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.