Okay, here we go.
Men of War: Awesome World War II RTT, but it can be a little impenetrable for some people, since the difficulty level is insane in some parts even when played on Easy.
IL-2 Sturmovik: Still, in my opinion, the best WWII combat flight simulator ever made, and it has cooperative multiplayer, which is always a plus.
Psychonauts: Fantastically fun puzzle-platformer that never received nearly as much publicity as it shoud have, since it's a fantastic game.
Grim Fandango: This is, in my opinion, the best adventure game ever made, and probably up there with the best written games ever.
Call of Duty: To be honest, I'd be surprised if there was anyone on here who hasn't played the first Call of Duty game, but if there are, you definitely should. It's worth it for either the first Russian mission or the American mission where you're careening around the countryside in a banged up citroen alone.
Deus Ex: The combat is awful, the mechanics are clunky, but the meaningful choices you have to make that affect the narrative and the crazy cyber-punk plot that reads like all the ridiculous fantasies of a first-year PolSci major mixed together make this a game you should totally play.
The Half-Life Series: Seriously, does this really even need to be said?