First Person Shooter/RPG with a crazy multi-pathed storyline and more conspiracy theories than you can shake a stick at, Including :D
-Aliens from the supposed Roswell crash
-MIB agents
-Nanoaugmentations (your character is a nano-aug agent)
-Illuminati trying to take over the world
-The Knights Templar having secret stashes of gold
-Area 51 (you go there)
-Massive corporations influencing government enough to control countries
-America monitoring all online activities and centralising all communications in the world
-Secret corporations trying to contol population numbers using a deadly virus
That's about it....although I probably missed something :P
If you haven't tried it I suggest giving it a shot. It'll run on most computers (as it's a bit dated now) and is only $10 now for the GOTYE version. It's so good it won 12 different GOTY awards!
Other than that:
Freelancer
Battlefield 1942 + Desert Combat (beats BF2 any day)
Dawn of War
GTA series
Postal 2 (that is one twisted game :crazy: )