Syncthing. Amazing tool. I keep media and documents syncted between two computers and a Pi home server. I want to add an "untrusted" VPS to the mix, but haven't done it yet. The only weak link is my iPhone. Luckily, one of the computer's in the mix is a Mac, and it keeps things synced via iCloud.

icloudpd. Pulls photos from iCloud onto the Pi and Syncthing takes care of it from there.

Prometheus/Grafana. Monitor indoor air quality with nice dashboards. I have other ideas I'd like to dashboard, but never get around to doing it.

Pi-hole.