I still play Elona and several flavors of Angband.

Greatly improved by community contributions.

If you bother to write a response, then please at least write what genre of game it is.

Cataclysm: Dark Days Ahead

It's a time machine that teleports you to 3am the next day.

Sanmill

Basically nine man's morris, it's pretty fun trying to beat progressively harder AI. Each difficulty requires a different (better) strategy. It's like unlocking levels in a puzzle game.

Quake

I'm a simple girl:

• LTris
• LBreakoutHD
• Gweled
• Frozen Bubble
• Gnome Mahjongg
• Gnome Tetravex

Too bad there isn't a single good linux pacman around...

Open Hexagon

Andor's Trail - RPG where you search for your missing brother. Still under development, but there's a lot of content. It's convenient to fill a few spare minutes or waste hours.

Apotris is an excellent famous-block-stacking-game clone for the GBA (and other platforms), it has a version for Portmaster that will run on many if not all Linux handhelds like the RG35XX and similar, but will also run in any GBA emulator.

Minetest and Widelands

From fdroid, Feudal Tactics and Antimine.

Technically Urban Terror isn't open source, but the Quake3 engine it runs on is.