What's Your "I Can't Believe This Is Free" Developer Tool?

Mine is Playwright . Free. Open source. Maintained by Microsoft. It does browser automation, testing, screenshot generation, PDF creation — and it handles dynamic JavaScript pages that Selenium chokes on. I use it to: Monitor competitor websites for price changes Generate screenshots of web pages for reports Automate form submissions Scrape JavaScript-heavy single-page apps Every time I set it up for a new project, I think "this should cost $50/month." Runner-ups: SQLite — a database in a single file. Powers more apps than any other DB. jq — JSON processing in the terminal. Use it 20+ times a day. ripgrep — searches 100GB codebases in seconds. Apify Free Tier — run web scrapers in the cloud, 5 sec/day free compute. What's yours? The tool that makes you think "how is this free?" Bonus points if it's something obscure that most devs haven't heard of.