15 Free Online Tools Every Developer Should Bookmark
A curated toolkit of free, browser-based developer utilities.
A big part of moving fast as a developer is not reinventing the wheel for small, repetitive tasks. You don't need a heavyweight app to format a JSON blob, decode a token, or generate a placeholder image — a good browser tool does it in seconds. Here are fifteen categories of free, online utilities worth keeping a click away, and what to look for in each.
I'm deliberately listing tools by category rather than pushing one specific brand — the category is what's durable. Many of these live in my own toolbox at tools.yydev.net, and countless open alternatives exist too.
Formatting & validation
1. JSON formatter & validator
Paste minified or broken JSON and get it pretty-printed with syntax highlighting and clear error messages pointing to the exact line that's malformed. Indispensable when you're staring at an API response that "should" be valid but isn't.
2. Code beautifier / minifier
Two sides of one coin: expand messy HTML, CSS, or JavaScript into readable form, or squash it down for production. Handy for inspecting someone else's compiled bundle or quickly shrinking a snippet.
3. Regex tester
A live regular-expression playground that highlights matches as you type and explains each token. Regex is famously easy to get wrong; testing against real sample text before you ship it saves hours of debugging.
Encoding & conversion
4. Base64 encoder / decoder
Convert text or files to and from Base64. Useful for embedding small images in CSS, inspecting data URIs, or decoding values you find in configs and headers.
5. URL encoder / decoder
Safely encode query parameters or decode that wall of %20s in a link. A must when you're building or debugging URLs by hand.
6. JWT decoder
Paste a JSON Web Token and see its header and payload decoded instantly. Perfect for checking what claims a token actually carries and when it expires — without writing a line of code.
7. Timestamp / epoch converter
Turn a Unix timestamp into a human-readable date and back again. If you've ever squinted at 1721001600 wondering what day that is, this one earns its bookmark.
Design & assets
8. Color picker & palette generator
Grab hex, RGB, and HSL values, check contrast ratios for accessibility, and generate harmonious palettes. Great for quickly building a colour scheme that doesn't fail WCAG contrast checks.
9. Placeholder image generator
Produce dummy images at any dimension for mockups and layout testing. Saves you from hunting down real assets while you're still roughing out a design.
10. Favicon generator
Upload a logo and get every favicon size and format a modern site needs, plus the HTML to drop into your <head>. A five-minute job that would otherwise be fiddly.
11. SVG optimiser
Strip the bloat that design tools leave in exported SVGs — often cutting file size by half or more without any visible change. Faster pages for free.
Testing & debugging
12. HTTP request tester
Fire off GET, POST, and other requests to an API and inspect the full response, headers included — no need to open a full API client for a quick check.
13. Lorem ipsum / mock data generator
Generate placeholder text, names, emails, and structured JSON records to populate a UI while you build. Real-looking data surfaces layout bugs that "test test test" never will.
14. Diff checker
Paste two blocks of text or code and see exactly what changed, line by line. Perfect for comparing config files or spotting what an AI assistant altered in a snippet.
15. Cron expression builder
Cron syntax is cryptic. A visual builder lets you describe a schedule in plain language and get the correct expression — and confirm when it'll next run.
How to actually use a toolbox like this
Two quick tips. First, bookmark a hub rather than fifteen separate pages — a single toolbox site keeps everything one tab away. Second, be mindful of privacy: for anything sensitive (production tokens, private keys, customer data), prefer tools that process everything in your browser rather than uploading to a server. Good utilities say so explicitly.
None of these tools will transform your career on their own — but together they remove dozens of tiny points of friction from your week, and that adds up to real time back. Keep the ones you reach for most within a click, and let the rest live in the toolbox until you need them.
That is my current shortlist. Tools come and go, so if there is one you swear by that I left out, let me know and I will take a look.
— yydev