Application Monitoring & Metrics
bugsnag.com
Free for up to 2,000 errors/month after the initial trial
CatchJS.com
JavaScript error tracking with screenshots and click trails. Free for open-source projects
Semaphr
Free all-in-one kill switch for your mobile apps
Jam
Developer friendly bug reports in one click. Free plan with unlimited jams
Whitespace
– One-click bug reports straight in your browser. Free plan with unlimited recordings for personal use
memfault.com
Cloud device observability and debugging platform. 100 devices free for Nordic, NXP, and Laird devices
exceptionless
Real-time error, feature, log reporting, and more. Free for 3k events per month/1 user. Open source and easy to self-host for unlimited use
skylight.io
Free for first 100,000 requests (Rails only)
Checkly
Code-first synthetic monitoring for modern DevOps. Monitor your APIs and apps at a fraction of the price of legacy providers. Powered by a Monitoring as Code workflow and Playwright. Generous free tier for devs
Sentry
Application monitoring for bugs and performance. 
sitesure.net
Website and cron monitoring - 2 monitors free
logentries.com
Free up to 5 GB/month with seven days retention