Also, here is a link to a list of other open source games on wikipedia, some are FPS's such as Doom and Duke Nukem 3D that started life as regular commercial software but are now open source. http://en.wikipedia.org/wiki/List_of_open_source_games
Although I may not have played some of them from looking at gameplay videos they seem great:
Batman Arkham Asylum
The Witcher
Portal(its a first person puzzler)
Vindictus(its basically the god of war of mmorpgs)
Street Fighter 4
Crazy Machines 2
Mirrors Edge(first person acrobatic game)
Shank
Amnesia the Dark descent
Penumbra
Trine
StarCraft 2
Mabinogi
Assassins creed
Yeah I've looked at NFS: HP before. How are the graphics? I didn't know you can earn enough points to open up paid parts, maybe I'll look at some MMO's.
The graphics are right up there with some of the best, imo. Check out some reviews and youtube vids if you're on the fence, but I think you'll dig it.
Aren't both of the fallout games technically FPS? Maybe FPS+. There are elements from many different genres, but I've always considered it FPS at it's most base. Either way, great games and highly recommended... if you've got the time to invest...
Well if the fallout is recommended then you can't pass Stalker. Similar FPS RPG shooter. I like Stalker over fallout, even thought fallout has a bigger world and longer campaign.
If you want something to last you, check out the DCS series
www.digitalcombatsimulator.com
It takes about a month just to learn to play, never mind play well. Top that off with countless user made campaigns & missions.