20 Free Developer Tools We Built (And Why We Gave Them Away)

You know those random utility websites you visit once, use for 30 seconds, and never think about again?

We built 20 of them. And put them all in one place at jo4.io/u.

No signup. No ads. No "subscribe to access." Just tools.

Why Build Free Tools on a Paid Product?

We're a URL shortener. People pay us for short links, analytics, and QR codes. So why give away free tools?

1. We Needed Them

This is the honest answer. While building jo4, we constantly needed:

• JWT decoder to debug OAuth tokens
• Base64 encoder/decoder for API payloads
• JSON formatter to read webhook responses
• Timestamp converter to debug expiration issues
• Hash generator for testing HMAC signatures

We were opening random websites, getting hit with ads and popups, and thinking "this is stupid." So we built our own.

2. We're Developer-Friendly

Our tagline is literally "Built for developers who ship." If we're going to claim that, we need to prove it.

Free tools with no signup, no dark patterns, no BS—that's what developer-friendly looks like. It's not just marketing. It's our identity.

3. SEO (Yes, Also This)

Every developer searching for "base64 decoder" or "jwt decoder online" is a potential customer who might also need a URL shortener. The tools get traffic, the traffic discovers our main product.

It's a legitimate marketing play. But the difference is: we built tools we actually use. Not half-baked "sign up to see results" garbage.

The Full List

Encoding & Decoding

JWT Decoder is our most-used tool. Paste a token, instantly see the header, payload, and expiration time. No external requests—everything happens in your browser.

Generators

Password Generator calculates actual entropy, not fake "strength bars." A 12-character password with all character types = ~79 bits of entropy. We show the math.

Text Tools

Diff Checker is surprisingly powerful. It auto-detects JSON/YAML and formats before comparing, so you can paste minified JSON and still get a readable diff.

Converters

Unix Timestamp Converter handles milliseconds vs seconds automatically. No more "is this 10 digits or 13?" confusion.

URL & Marketing Tools

UTM Builder ties directly into our URL shortener. Build your UTM link, optionally shorten it, track everything. Full funnel in one page.

Design Philosophy

1. Client-Side First

Most tools run entirely in your browser. No server requests. No data sent anywhere. When you paste a JWT, it never leaves your machine.

The only tools that require API calls:

• URL Checker (needs to fetch the actual URL)
• OG Preview (needs to fetch metadata)

And even those are rate-limited and don't store anything.

2. No Dark Patterns

• No "sign up to see results"
• No "share to unlock"
• No interstitial ads
• No newsletter popups
• No fake urgency

You land on the page, use the tool, leave. That's it.

3. Keyboard-First

Every tool supports:

• Ctrl/Cmd + Enter to execute
• Ctrl/Cmd + C to copy result
• Esc to clear

We use these tools daily. Keyboard shortcuts matter.

4. Mobile-Responsive

All tools work on mobile. The text areas resize. The buttons are tap-friendly. You can decode a JWT on your phone at 2 AM when production is on fire.

Not that we've ever done that.

The Tech Stack

React + TypeScript + Tailwind CSS
├── Client-side crypto for hashes
├── Client-side JWT parsing
├── Client-side QR generation
├── RTK Query for API calls
└── Shared UI components (shadcn/ui)

Each tool is a standalone page component. No shared state. No complex routing. Load fast, do one thing well.

Usage Stats (The SEO Payoff)

After 3 months:

Total: ~15,000 monthly visitors who now know jo4.io exists.

Conversion to paid? Low, around 0.3%. But that's 45 paying customers who found us through free tools. At $16/month average, that's $720 MRR from SEO content that costs us nothing to maintain.

Tools We're Still Building

The /u/* pattern works. We're adding more:

• Cron Expression Builder - Build and explain cron expressions
• Regex Tester - Test regex patterns with live matching
• HTTP Header Inspector - See request/response headers for any URL
• SVG to PNG Converter - Convert SVG files to raster images

If you have suggestions, let us know.

The Full Directory

Everything lives at jo4.io/u. Bookmark it. Use it. Tell your friends.

20 tools. Zero signup. Zero cost.

What tools do you wish existed? We're always looking for ideas.

Building jo4.io - URL shortener with analytics, plus 20 free developer tools at jo4.io/u.