Years ago my "dev tools" bookmark folder had about twenty entries, each a different single-purpose website, each with its own ad layout, its own cookie banner, and — this is the part that finally bothered me — its own server that my data was quietly being uploaded to. I pasted a staging API token into one of them to decode it, and halfway through I thought: I have no idea what this site does with that string. That was the day I stopped trusting random tool sites and started building toolz.dev, where every tool runs in your browser and nothing you paste leaves the tab.
This is the toolkit that came out of that. Not a "top 600 tools" listicle — toolz.dev has just over fifty tools right now, and I would rather show you the ones a working web developer opens every week than pad the count. They are grouped here by the job you are doing: shaping data, wrangling text, writing CSS, prepping images, shipping SEO, and the security-and-encoding chores in between. Everything is free, needs no account, and processes locally.
TL;DR: Keep one browser tab group open while you build. For data: JSON Formatter, JSON Diff, YAML Validator, CSV Viewer. For debugging tokens: JWT Decoder, Base64 Converter, Timestamp Converter. For regex: Regex Tester. For CSS: Gradient Generator, Color Picker, CSS Minifier. For images: Resize, Compress, Format Converter. For SEO: Meta Tag Generator, Open Graph Preview. All client-side, all free.
Why Bother With Browser Tools When You Have a Terminal?
Fair question — I live in a terminal too. The honest answer is that for one-off, look-at-it tasks, a visual tool with instant feedback beats remembering the jq incantation or the openssl flag. When an API hands me a minified blob at 2 a.m., I do not want to write a formatting script; I want to paste it and see it. When I need to eyeball whether two JSON responses differ, a red/green diff is faster than piping through anything.
But the deciding factor is not convenience, it is where the data goes. A huge number of "free online" tools upload whatever you paste to a server to process it. That is fine for a public sample and a genuine problem for a production token, a customer record, or an unreleased asset. Every tool in this guide runs entirely in your browser — the processing happens in JavaScript on your machine, and there is no upload step to worry about. That combination, visual-and-instant plus private-by-default, is the whole reason to keep these in a tab group instead of shelling out.
Shaping Data: JSON, YAML, CSV, XML
This is the category I use most, because most of web development is moving structured data between systems and squinting at it when it breaks.
The JSON Formatter is the workhorse — paste a minified API response and get indented, readable output with the syntax error located when the JSON is invalid. When you need to know what changed between two payloads, the JSON Diff compares structurally rather than line-by-line, so reordered keys do not create phantom differences. For the config side of the job, the YAML Validator catches the quiet YAML traps — tabs in indentation, duplicate keys, and the type-coercion surprises that turn version: 1.10 into the number 1.1 — before a Kubernetes or CI file blows up a deploy.
Data rarely stays in one shape. The JSON to CSV and CSV to JSON converters move between API output and spreadsheets, the CSV Viewer renders a messy CSV as a real table so you can actually read it, and the XML to JSON and JSON to YAML tools cover the format hops you hit with legacy APIs and config files. If you want the deeper version of this category, the complete guide to JSON tools walks through the whole workflow.
Debugging Tokens and Time: JWT, Base64, Timestamps
Auth and time are where "the code looks right" bugs come to hide, and they are almost always solved by looking at the data instead of re-reading the logic.
The JWT Decoder splits a token into header, payload, and signature and shows you the claims — which is how you discover that exp you swore was fine is actually in the past. Reach for it instead of a generic decoder, because JWTs use base64url (RFC 4648 §5), not standard Base64, and a plain Base64 Converter is the right tool for everything else — Basic auth headers, data URIs, encoded payloads. The Timestamp Converter turns Unix seconds and milliseconds into real dates (the seconds-vs-milliseconds mix-up is the single most common auth bug I see), and the Timezone Converter and Date Difference tools handle the "what time was this for the user" and "how long between these two events" questions. I wrote up the whole debugging flow in the API debugging tools guide if you want the war stories.
Regex Without the Guesswork
Regular expressions are write-once, debug-forever. The Regex Tester lets you build a pattern against sample text with live match highlighting, so you see what matches the moment you type it instead of deploying a validator and hoping. If you would rather assemble a pattern from building blocks, the Regex Builder approaches it from the other direction. Either way, the point is the same: test the pattern against real strings before it guards a form or parses a log. The regex builder guide covers the common patterns worth memorizing.
Text Wrangling: Counting, Casing, Diffing, Placeholder
The unglamorous string chores add up. The Word Counter checks content against length limits (meta descriptions, tweets, headings). The Case Converter flips between camelCase, snake_case, kebab-case, and PascalCase — invaluable when you are porting variable names across a codebase. The Text Diff compares two blocks and highlights every change, which is code review outside of Git and config auditing rolled into one.
For building and prototyping specifically: the Lorem Ipsum Generator fills mockups with the right amount of placeholder text (there is a full guide to sizing placeholder text), the Slug Generator turns titles into clean URL paths, and the String Reverse tool handles the odd one-off. The text tools guide ties these together.
CSS and Color
Front-end polish lives here. The Gradient Generator gives you a visual editor for linear and radial gradients and hands back production CSS — no guessing at angles and stops (the gradient guide goes deeper). The Color Picker converts between HEX, RGB, and HSL and is where I check contrast before a color ends up failing an accessibility audit; the color picker guide covers the contrast math. When it is time to ship, the CSS Minifier and HTML Minifier strip whitespace and comments to shave bytes off the wire.
Images for the Web
Images are usually the heaviest thing on a page, so this category pays for itself in Core Web Vitals. The Image Resize tool sets exact dimensions (and there is a retina trap worth knowing about, covered in its guide), the Image Compressor shrinks file size while holding quality, and the Image Format Converter moves between PNG, JPG, and WebP so you serve the right format for the job. The Background Remover is a color-key remover — fast and private, great on flat backgrounds, and honest about its limits in its guide. The image tools guide is the pillar for this set.
Shipping: SEO, Sharing, and Security
The last mile before a page goes live. The Meta Tag Generator builds your title, description, Open Graph, and Twitter card tags, and the Open Graph Preview shows how the link will look when it is shared before you find out the hard way on social. The Robots.txt Generator and UTM Builder handle crawl rules and campaign tracking, and the QR Generator makes a scannable code for print or slides. On the security-hygiene side, the Password Generator and Hash Generator cover strong secrets and integrity checks, and the UUID Generator spits out identifiers for test data and database keys.
Client-Side Tools vs. Upload-Based Tools: The Comparison That Matters
| toolz.dev (client-side) | Typical "free online tool" | |
|---|---|---|
| Where processing happens | In your browser | On a remote server |
| Your data (tokens, files, code) | Never leaves the tab | Uploaded to a third party |
| Signup / login | None | Often required for full features |
| Usage limits | None on the tools | Frequently capped or paywalled |
| Speed | Instant, no round trip | Depends on upload + server |
| Works with sensitive data | Yes, by design | Risky — you are trusting their server |
| Number of tools | Just over fifty, curated | Usually one tool per site |
The row I care about most is the second one. For a public sample it does not matter where processing happens; for a production secret it matters enormously. Building the whole toolkit client-side means I never have to ask "is this the safe site to paste this into" — they all are.
How I Actually Use This: Chain the Tools
Most real tasks are a small pipeline, not a single click:
Debugging an API response: paste it into the JSON Formatter, decode any token with the JWT Decoder, turn the exp claim into a date with the Timestamp Converter, then compare the broken response against a working one with the JSON Diff.
Prepping a hero image: Background Remover if it needs a cutout, then Image Resize for exact dimensions, then Image Compressor to hit your page-weight budget, converting to WebP with the Format Converter on the way out.
Shipping a new page: generate tags with the Meta Tag Generator, check the share card in the Open Graph Preview, and confirm your copy fits with the Word Counter.
Keep the five or six you touch daily in a pinned tab group. The rest are one search away in the ⌘K palette when you need them.
Frequently Asked Questions
Are these web developer tools really free?
Yes. Every tool on toolz.dev is free to use with no premium tier, no feature gate, and no usage cap on the tools themselves. There is no account required to use them.
Do I have to create an account or log in?
No. Open a tool and use it immediately. There is no signup, and because the tools run in your browser there is nothing to sync to a profile.
Is it safe to paste tokens, keys, or private data?
Yes, and that is the core design decision. The tools process everything client-side in JavaScript, so what you paste — a JWT, an API response, an unreleased image — is handled on your machine and is not uploaded to a server. That is the main reason to use them over upload-based alternatives.
How many tools does toolz.dev have?
Just over fifty at the moment, spanning data, text, coding, CSS, color, image, SEO, security, and calculator categories. I would rather grow the set deliberately than claim a number that is mostly filler — every tool listed here exists and works today.
Do the tools work on mobile?
Yes, the interfaces are responsive and work in mobile browsers. Heavier jobs — large image processing especially — are faster and more comfortable on desktop, but the tools themselves run either way.
Do these tools work offline?
Mostly yes. Because each tool runs in JavaScript in your browser, a tool keeps working with no connection once its page has loaded — useful on a plane or a flaky network. You only need to be online to open a tool the first time; after that the processing itself needs no server.
What is the difference between client-side and server-side online tools?
A client-side tool does its work in your browser, so the data you paste never leaves your machine. A server-side tool uploads your input to a remote computer to process it and sends the result back. The output can look identical, but only client-side tools keep a production token, private file, or customer record off someone else's server — which is why every tool here is built client-side.
Can I use these instead of my terminal or IDE tooling?
For one-off, visual tasks — formatting a blob, decoding a token, eyeballing a diff, building a gradient — they are often faster than remembering the command-line equivalent. For scripted, repeatable work you will still want your terminal. In practice they sit side by side: fire the request in your usual client, then paste the output into these to read it.
Which tools should I bookmark first?
Start with the ones you will touch daily: the JSON Formatter, JWT Decoder, Regex Tester, Timestamp Converter, and whichever of the CSS or image tools matches your work. Add the rest as you hit the tasks that need them.
Wrapping Up
A modern web developer's toolkit is not one big app — it is a dozen small, sharp tools you reach for without thinking. The trap is scattering them across a dozen sketchy websites that upload your data to process it. The fix is keeping the ones you trust in a single tab group, all running locally, all free, all one keystroke away.
That is what toolz.dev is: just over fifty of those tools, built client-side so your tokens and files stay yours. Pin the JSON Formatter, the Regex Tester, and the Timestamp Converter today, and add the rest as the work demands. Free forever, private by design.
