The free online developer tools worth bookmarking in 2025
A curated list of free browser-based developer tools that run entirely client-side - no sign-in, nothing sent to a server. The ones actually worth keeping in your bookmarks.
Most developer tools on the web are one of two things: a frontend that ships your data to a server, or a polished SaaS product that wants your email before showing you anything. Neither is what you want when you're pasting a JWT from a production environment, or formatting a JSON blob that might contain PII.
The GKit tools collection takes a different approach. Every tool runs entirely in your browser - no requests leave your device, no account required. The JavaScript executes locally, and closing the tab is as close to a guarantee as you can get that nothing was retained. That matters more than it sounds when you're working with tokens, keys, hashes, or anything you'd hesitate to paste into a chat window.
Here's what's in the collection and what each tool actually does.
JSON tools
JSON is the format developers interact with most. These tools cover the full range of tasks that come up when you're debugging an API response, comparing two objects, or trying to make sense of a deeply nested structure.
JSON formatter
The JSON formatter pretty-prints minified JSON into indented, readable output, validates syntax and reports the exact line and character of any error, and can minify a formatted blob back to a single line. Useful when an API returns a wall of text and you need to read it.
JSON diff
Paste two JSON objects side by side and the diff tool highlights exactly what changed: added keys, removed keys, changed values. It handles nested objects and arrays, and shows the structural delta rather than a raw text diff. More reliable than eyeballing two objects when the structure is non-trivial.
JSON flattener
Converts a deeply nested JSON object into a flat key-value map using dot-notation paths - user.address.city instead of three levels of nesting. Useful for understanding the full key surface of a complex object, or for feeding nested data into systems that expect flat structures.
CSV to JSON
Converts a CSV file or pasted CSV text into a JSON array, using the first row as field names. The CSV to JSON converter handles quoted fields, commas inside values, and common encoding edge cases. Also converts in the other direction - JSON arrays back to CSV - for when you need to hand data off to a spreadsheet.
JSON path tester
Test a JSONPath expression against a live JSON object and see the matching nodes highlighted in the tree. Useful for verifying the path you're about to hardcode actually selects what you think it does.
JSON schema validator
Paste a JSON schema and a JSON object, and the validator reports whether the object conforms - including which properties failed and why. Saves the round-trip of deploying code just to see a validation error.
Auth and security
These are the tools where client-side execution matters most. You should not paste a JWT or run a hash through a tool that phones home.
JWT decoder
The JWT decoder splits a token into its three parts, base64-decodes the header and payload, and displays the claims in readable JSON - including the expiry timestamp as a human-readable date. It does not verify the signature (that requires the secret), but it shows you everything the token contains. Nothing leaves the browser.
Hash generator
The hash generator computes MD5, SHA-1, SHA-256, and SHA-512 hashes of any input string. Useful for verifying file integrity, generating checksums for API request signing, or checking that two strings produce the same digest. All computation runs via the Web Crypto API in the browser.
Password generator
Generates cryptographically random passwords with configurable length, character sets (uppercase, lowercase, numbers, symbols), and minimum counts per character type. Uses crypto.getRandomValues - the same source of entropy your browser uses for TLS.
Base64 encode/decode
The base64 tool encodes arbitrary text or binary to base64, and decodes it back. Handles standard base64 and URL-safe base64 (the variant used in JWTs and some API parameters). Also decodes base64-encoded JSON inline, which saves a copy-paste step when you're reading encoded payloads.
API development
Tools for the part of the job that involves HTTP - building requests, understanding responses, and decoding the various formats that APIs use.
curl builder
Builds a curl command from a form - method, URL, headers, body, auth. The curl builder handles Bearer tokens, Basic auth, and custom headers without you having to remember the exact flag ordering. Useful for constructing the curl equivalent of a request you're about to test, or for generating a shareable reproduction case.
HTTP status codes reference
A searchable reference for every HTTP status code - 1xx through 5xx - with the official name, a plain-English description of what it means, and when it's appropriate to return it. Faster than a browser search when you need to remember what 422 means versus 400.
HTTP headers reference
Covers request and response headers with descriptions of what each one does, valid values, and common usage. Includes security headers (Content-Security-Policy, Strict-Transport-Security), caching headers, and CORS headers. The kind of reference you want open while you're writing middleware.
MIME types lookup
Search by file extension to get the correct MIME type, or search by MIME type to find what extensions it covers. Useful when you're setting Content-Type on a response and want to be certain you have the right string.
URL encoder/decoder
The URL encoder percent-encodes a string for safe use in a URL parameter, and decodes percent-encoded strings back to readable text. Handles both encodeURIComponent and encodeURI semantics.
URL parser
Breaks a URL into its component parts - scheme, host, port, path, query string, fragment - and displays each one separately. Useful when you need to quickly read a deeply nested URL without manually scanning through it.
Query string parser
Parses a query string into a key-value table, handling repeated keys, encoded characters, and nested bracket notation (filter[status]=active). Paste the raw query string or the full URL.
Time and scheduling
Timestamps and cron expressions are two of the most error-prone things to write by hand. These tools make both of them legible.
Unix timestamp converter
The timestamp converter converts Unix timestamps (seconds or milliseconds) to human-readable dates in any timezone, and converts dates back to timestamps. Shows the value in UTC, local time, and a selectable timezone simultaneously. Useful when you're reading log output or an API response and need to know what a number actually means.
Timezone converter
Pick a time in one timezone and see it in several others simultaneously. Useful when you're scheduling something across distributed teams and want to verify the time makes sense in each location before sending a calendar invite.
Duration calculator
Add or subtract durations from a timestamp, or calculate the duration between two timestamps. Returns the result in multiple units - days, hours, minutes, seconds - simultaneously.
Cron builder
The cron builder generates a cron expression from human selections - minute, hour, day of month, month, day of week - with presets for common patterns (every 5 minutes, daily at midnight, every weekday). Eliminates the need to remember which field controls which unit.
Cron explainer
Paste a cron expression and get a plain-English explanation of when it fires, a list of the next ten scheduled run times, and an indication of whether it would run at a specific moment you provide. Useful for verifying that a schedule you inherited actually does what you think.
Text and data
General-purpose text processing tools that come up in writing, documentation, and data cleaning work.
Regex tester
The regex tester runs a regular expression against input text and highlights every match inline. Shows capture groups and their values, supports flags (global, case-insensitive, multiline, dotAll), and updates in real time as you type. Faster than the edit-run-print cycle in a script.
Text diff
Paste two blocks of text and see a line-by-line diff with added and removed lines highlighted. Works on code, prose, configuration files - anything textual.
Word counter
Counts words, characters (with and without spaces), sentences, paragraphs, and lines. Also shows estimated reading time. Useful when you're writing for a platform with a character limit and want to check before submitting.
Readability scorer
Calculates Flesch-Kincaid reading ease, grade level, and a few other readability metrics for a block of text. Useful for documentation, marketing copy, or anything where you want to calibrate complexity for an intended audience.
Markdown to HTML
Renders Markdown to HTML and shows the output side by side. Useful for previewing how content will render before deploying it, or for converting Markdown to HTML for use in contexts that don't render Markdown natively.
Markdown table generator
A table editor that generates the correct Markdown table syntax - column widths, alignment markers, escaped pipes. Eliminates the tedium of hand-aligning Markdown tables.
Lorem ipsum generator
Generates placeholder text in paragraphs, sentences, words, or bytes. Standard lorem ipsum or randomized. Useful during prototyping when you need realistic-looking text volume to evaluate layout.
Case converter
Converts text between camelCase, PascalCase, snake_case, kebab-case, SCREAMING_SNAKE_CASE, Title Case, and sentence case. Handles the edge cases that manual search-and-replace misses.
String escape
Escapes and unescapes strings for JSON, JavaScript, HTML, SQL, and URL contexts. Paste a string containing quotes or special characters and get the correctly escaped version for the target context.
Numbers and encoding
UUID generator
The UUID generator generates version 4 UUIDs using the browser's crypto.getRandomValues. Generate one or a batch, copy individually or all at once. Useful when you need IDs for database seeds, test fixtures, or mock data.
Number base converter
Converts integers between binary, octal, decimal, and hexadecimal. Shows all four representations simultaneously and updates in real time. Useful when reading bit flags, color values, or memory addresses.
Binary viewer
Shows the binary, octal, decimal, and hex representations of each byte in an input string, with the option to display as ASCII or UTF-8 codepoints. Useful for debugging encoding issues or understanding how a string is actually stored.
Statistics calculator
Given a list of numbers, computes mean, median, mode, range, variance, standard deviation, min, max, and sum. Paste a column of values from a spreadsheet and get the full summary instantly.
Semver parser
Parses a semantic version string into its components (major, minor, patch, prerelease, build metadata) and validates whether it conforms to the semver spec. Also compares two semver strings and tells you which is greater.
Color and design
Color contrast checker
Checks the contrast ratio between a foreground and background color against WCAG AA and AAA thresholds - for both normal text and large text. Shows the exact ratio and whether each size/level combination passes. Useful during design review and before shipping any UI.
Color converter
Converts color values between hex, RGB, HSL, HSV, and CSS named colors. Shows all representations simultaneously and lets you adjust any of them with instant conversion.
Color palette generator
Generates complementary, analogous, triadic, and split-complementary palettes from a base color. Shows each palette with hex values and allows exporting as CSS custom properties. Useful for building a color system from a single brand color.
Network tools
IP info lookup
Enter an IP address and get its geolocation (country, region, city), ASN, and organization. Useful for sanity-checking where traffic is coming from in server logs.
CIDR calculator
Takes a CIDR notation block (e.g. 192.168.1.0/24) and returns the network address, broadcast address, subnet mask, first and last host addresses, and total host count. Useful when configuring VPCs, firewall rules, or any network that uses subnetting.
The privacy angle
The reason to care about client-side execution is not performance or convenience - it's that developer tools routinely handle sensitive material. JWTs contain user identities and scopes. JSON payloads from internal APIs contain schema information. Hashes may be computed against passwords or keys. A regex test might be run against a production log excerpt.
When a tool runs in the browser, the data you paste into it never reaches a server. There is no request to log, no session to tie your input to, no operator who could be compelled to produce records of what you typed. You're using JavaScript that was served to your browser once; everything after that is local.
All GKit tools work this way. No sign-in, no tracking, no server-side processing of your input.
The rest of GKit
The tools collection is the free, no-account layer of GKit. The platform also includes products for teams that need something more structural:
SheetsAPI turns a Google Sheet into a JSON API endpoint - with filtering, pagination, sorting, and API key authentication. For when you need Sheets as a lightweight data layer without building OAuth infrastructure yourself.
Drive Cleaner finds duplicate files, orphaned items, and space hogs across a Google Drive, and lets you clean them up in bulk. For when your Drive has grown large enough that manual cleanup is no longer practical.
Both products connect through a single Google account via the GKit dashboard. The tools collection needs nothing at all - just a browser.
Browse the full tools collection and keep the ones you reach for.