What's the one free tool you keep going back to — and what made it stick?

I'm launching bypassed.me here tomorrow — 169 free DNS, email, TLS and dev diagnostics in one place. It started as a personal toolbox for debugging my own self-hosted mail server and grew a bit out of hand 😅

While building it, I noticed the tools I personally return to aren't the fanciest ones — they're the ones that answer one question instantly with zero friction (for me: a cron-to-plain-English translator and a cert-chain checker).

So I'm curious what that tool is for you — the free one that's earned a permanent bookmark, and what it got right. Partly because I'm genuinely interested, partly because the answers will tell me what to build next. What's missing from every toolbox you've tried?