In no particular order:
Half-Life (the original)
Half-Life 2 & Episodes (even greater than the original)
Portal (short but oh so sweet!)
Deus Ex
System Shock 2 (arguably my all time favorite, I no longer like monkeys)
Giants: Citizen Kabuto (hilarious and original)
F.E.A.R. & 1st Expansion (skip the 2nd expansion)
Unreal
Call of Duty 1, 2 & 4
No One Lives Forever Series (definitely up there with the favorites)
Bioshock (good thing: a lot like SS2, bad thing: too much like SS2)
Far Cry
Crysis & Expansion
Outlaws (oldie but goodie from Lucasarts)
Call of Juarez (underrated in my opinion)
Doom 3 & Expansion
Quake 4
Return to Castle Wolfenstein
Serious Sam
Serious Sam: The Second Encounter
Serious Sam 2 (not as good as the originals, but still fun)
Medal of Honor Series (the first ones are better than the latter ones)
Tribes: Vengeance
Star Wars Jedi Knight II: Jedi Outcast
Star Wars Jedi Knight: Jedi Academy
The Chronicles of Riddick: Escape from Butcher Bay
Soldier of Fortune I & II
Brothers in Arms Series
S.T.A.L.K.E.R. Shadow of Chernobyl (buggy but brilliant)
Max Payne I & II (bullet time FTW!)
Tom Clancy's Rainbow Six Vegas I & II
Freedom Fighters (whenever I play this I have to immediately play it a 2nd time)
Thief Series
Clive Barker's Undying
Splinter Cell Series
Aliens vs Predator 2
Gears of War