Does forkyz count? I got addicted into crosswords recently, and this one is nice.

Luanti a game engine that runs an Open Source massively moddable Minecraft clone, among other games.

Feudal Tactics, conquer-the-world strategy game with randomly-generated levels

Shattered Pixel Dungeon, popular roguelike (as in, actually like Rogue)

Rabbit Escape, basically Lemmings with cute bunnies. Don’t let them splat!

Xeonjia: Ice Adventures, extremely cute sliding-ice puzzle game

Apotris is probably the best version if tetris available, and it’s foss.

You can get a compiled ROM for GBA and play it on android with a FOSS emulator like Lemuroid. It works!