I already use Tubular, and I have been using NewPipe (or the fork Tubular) since the very first alpha.
Grayjay doesn’t work well for me since I’m subscribed to over 1000 creators and they force rate limiting over 200, which makes it completely unusable for my usecase.
DuckStation recently changed to a source-available license that prohibits distributing modified versions of the software and prohibits commercial use. Before, it was GPLv3.
Also OpenOffice, Emby, Audacity, Android (AOSP) (soft forked to LineageOS and GrapheneOS, but no hard fork)