Ok. Since there weren't any more answers for a while, let me summarize: If I list all games mentioned more than one time I get the following list:
The Fab 5 of libre gaming:
- Minetest
- 0 A.D.
- Super Tux Kart (drawback: unfree assets)
- Battle for Wesnoth
- Pingus
(In no particular order)
Edit: I personally think Pingus isn't so popular.