For fun, playing with Meshtastic https://meshtastic.org/ and contributing to the open source firmware and apps. They have something cool but need lots of help. I've patched 3 memory leaks and had a few other PRs merged already.

For work, https://heyoncall.com/ as the best tool for on-call alerting, website monitoring, cron job monitoring, especially for small teams and solo founders.

I guess they both fall under the category of "how do you build reliable systems out of unreliable distributed components" :)