Free dev tools that run entirely in your browser (no uploads, no signup)

I have a bad habit of opening the same five browser tabs every time I work with JSON, want a strong password, or need to compress a screenshot before a PR. For the longest time those tabs were a mix of different sites — some behind signup walls, some that upload whatever you paste, a few with more ads than actual functionality.

So I built a small collection of my own. It started with a JSON formatter and a character counter, and slowly grew into FreeToolsy — 90+ free tools for text, security, developer work, images and calculations.

The one rule that matters: everything runs in your browser. Nothing you paste or upload ever leaves your device, and there's no account to create.

The ones I actually use daily

JSON & data

• JSON Formatter — validates and prettifies JSON, points at the exact line of a syntax error. This is the tab that never closes.
• JSON to CSV — when someone puts 10k rows of API response in a ticket and asks for "an Excel file".
• JWT Decoder — decode the token, eyeball the claims, no need to trust a third-party site with your token. Perfect for local dev.
• Unix Timestamp Converter — the "what the heck is 1982880000" tool.
• Base64 — encode/decode with the whole format table, handy for data URIs.

Security

• Password Generator — customizable length and character sets, generated locally.
• Strength Checker — the "nope, don't use that" second opinion before you reuse a password.
• SHA Hash Generator — MD5/SHA-1/SHA-256/SHA-512, hashed client-side.

Text & clean-up

• Duplicate Line Remover — sorting a messy list of IDs or package names.
• Text Diff — quick "what changed between these two configs" check without spin up a diff tool.
• Slug Generator — before writing a new route or blog post.
• Regex Tester — match, explain and test against a sample live.

Scheduling & config

• Cron Tester — the one I reach for most when a background job "runs sometimes": expands the schedule into human-readable timestamps.
• AI Token Counter — quick estimate of prompt cost before you hit the API.
• WCAG Contrast Checker — because "it looks fine" is not an arbitrary standard.

Images

• Image Compressor — shrink a JPEG/PNG/WebP before pushing it into a PR. Nothing gets uploaded.
• Image Resizer — resize to exact px or by percentage.
• Image to Base64 — for inlining small assets directly into code.

Why "runs in your browser" is the whole point

A paste-anything-anywhere site is fine for throwaway data, but not for tokens, API keys, private configs or employee data. For a dev workflow, client-side execution changes the trust equation: no server logs, no retention policy to think about, no "we'll store this for analytics". It also means the thing still works on a flaky connection or an air-gapped machine — offline-friendly since most logic is plain JS.

What's next

I keep adding tools as I run into boring repetitive jobs, and each one includes a short guide. If you have a tool you use every single day or a gripe about the collection, I'd genuinely appreciate a suggestion — I'd rather build what people actually miss than a hundred tools nobody opens.

The whole thing is free and open to use: FreeToolsy. And if you want the reasoning behind one of the tools, the guides section covers things like what a QR code actually is and how JSON formatters structure data.