forty_sham (In no particular order.) Minetest Pingus Hedgewars SuperTux Endless Sky (Honorary mention: GL-117, a true classic.)
Technopeasant My favourite libre games are Freedoom, Avoision, Adventures on Planet Zephulor, Which Way Is Up, Pingus, Dave Gnukuem, Heroine Dusk, Trackballs, Me and My Shadow, XEvil, 0 A.D., Duckmaze, Orbit-Hopper and Xonotic.
poVoq Only libre assets makes it a bit harder. I tried to think of some less commonly mentioned ones aside from Veloren. Veloren Tanks of Freedom (also the new sequel) Block Attack Yorg Unciv
dulsi Mojotron Alchemy Quest (or Open Alchemist) Angband (without sound unfortunately) Smalltrek Cendric Inertia Blast
Modanung @dulsi Silent Angband or it doesn't count. ;) Cendric seems to have unfree assets as well. @Technopeasant That's not how you count to five! More like schfifty-five.
dulsi Modanung Sorry about that I didn't go back and check Cendric. Also didn't realize Angband sounds had a problem. Added Inertia Blast as a replacement for Cendric.
Technopeasant Modanung Fine, if you insist. 😛 Freedoom Adventures of Planet Zephulor Which Way Is Up Me and My Shadow Avoision
Modanung ...let me demonstrate once more: Powermanga Kobo Deluxe Frozen Bubble Crack Attack! Extreme Tux Racer EDIT: Frets of Fire needs a default free culture music pack. Licks of Liberty?
Peter 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.
forty_sham Peter I personally think Pingus isn't so popular. Actually, it was pretty high-ranking on Ubuntu Software Center for a period of time, and it still retains a 4-star rating.
Technopeasant It was also the first Game of the Month selected for work on The Linux Game Tome back in 2003. Seems like a fairly good and varied list.
[deleted] My top five would be: wesnoth red-eclipse 1 (not too sure for RE2) widelands endless sky (this game is still a bit young, but I really liked playing it) openclonk