Games originating in modding communities:
- 0ad
- SpringRTS
- OpenRA (total conversion mods required)
- OpenTTD
- The Dark Mod (Mod for DOOM 3, but is not FPS)
Games that are also sold on app stores, steam etc:
- shattered pixel dungeon
- mindustry
- keeperrl (only ascii version is free and I don’t know how playable it is in that state)
Games that are around for quite some time or gained quite a community around it at some point:
- The Battle of Wesnoth
- Cataclysm: Dark Days Ahead
- BrogueCE (animated ascii graphics)
- Minetest
- Super TuxKart
- Super Tux
- Hedgewars
Open sourced commercial games:
- Castle Doctrine
- Warzone 2100
- Soldat
- Astromenace
Though for those who are more engines (SpringRTS and minetest) the quality really depends on the mods you are playing.
Some stuff I just found and never played myself:
- Catburglar
- Roboden
Open sourced commercial games:
- Charge Kid
- duelyst
- Super Lemonade Factory
- OpenClonk
- Seven Kingdoms
Assets are unfree but freely accessible:
- Cendric2 (nc-nd)
- Star Ruler (nc without music)
- Cart Life (freeware)
- Postal (freeware)
- Pocket Island (nc-sa)
- Strange Adventures in Infinite Space (nc)
I’m not sure about whether these games got 100% FLOSSed or still require bought assets:
- BYTEPATH
A special case because these use CC BY-NC-SA even for source code, which is effectively unfree. They are ports of older Mac games, but most are 3D:
- Mighty Mike
- Cro-Mag Rally
- Bugdom 1
- Bugdom 2
- Billy Frontier
- Nanosaur 1
- Nanosaur 2
- Otto Matic
The “problem” with the open source game landscape is, that a lot of games are either focused on multiplayer or have randomly generated worlds, because that developers can play that too. There are games with single player story line, I think open sourced commercial games are doing a bit better with this. Commercial open source games that are open source from the beginning are a newer development.