There is the awesome self hosted list and sites like selfh.st

F-Droid is great to find open source Android programs. And for general open source software just installing any Linux distribution and looking in its software store equivalent will give you plenty of choices.

https://libregamewiki.org/Roguelike_games

There are more games on there, but I have been meaning to try the roguelike stuff out.

Flathub / Search is good for exploration in my opinion. I even found programs there, just to install it through a different package manager. If you are looking for FOSS only, just make sure the license is Open Source compatible. There is an icon on each app page you can click for more information:

Edit: You can apply a filter in the Search to show Open Source licensed projects only.

Host your projects in codeberg.org

Inkscape for graphic design