Oblivion is great. The encumberance system really hampers the heavily armored solder type character, though, if you go theif/assassin it isn't really an issue.
Alpha Centauri is an awesome game.
America's Army is a great online shooter, and it's free. more tactics than most other shooters.
Deus Ex is ridiculously cool.
Chronicles of Riddick: Escape from Butcher Bay or whatever is a pretty fun game. Good voices, fair amount of humour, and good gameplay.
Homeworld 1 and 2 are both great RTS games. Good online play, too.
If you've got a longer attention span try IL-2: Forgotten Battles + Ace Expansions Pack + Pacific Figthers + Pe2 Expansion pack. God that's a long list, they might sell it all bundled somewhere. Fantastic WWII flight sim. Great graphics, beautifully modeled cockpits, extremely accurate flight models and physics engine, great realism.
And awesome online play. So much more fun online than a shooter. Servers vary from realistic mission based servers to "airquake" servers (ridiculous good fun, those) to racing servers to dualling servers. I usually fly the "airquake" style servers as I'm not looking to spend several hours in a mission when I go online - that's what single player is for.