Software

GrapheneOS
I primarily use an iPhone these days, but this is undoublty the best operating system for a mobile device. Completely open source, hardened, and with sandboxed Google Play services.

Bitwarden
A free and open source cloud based password manager that can be selfhosted. Much better than LastPass or... God forbid... all your passwords in a browser!

DW Service
A free, open source remote viewing service that I like for monitoring and controlling machines, espeically family members' that I am responible for maintaining.

FreeFileSync
Windows Only
FOSS local file syncing.

Stats for MacOS
During my (technically second) experimentation with MacOS, I have found this piece of software extremely useful. It is much nicer for monitoring system resources than the actual Activity Monitor.