HALO 2.
http://halo.bungie.org/misc/halo2trailermirrors.html
The trailer is awesome. You will need Media Player 9 though, but it's worth the download.
My pick of the litter, in no particular order:
Raven Shield
Doom 3
G.I. Combat
Star Wars Galaxies
Age of Mythology
UT 2003
Unreal 2
FreeLancer
Planetside
World of Warcraft
Project Ego
Splinter Cell
C&C Generals
Deus Ex 2
No One Lives Forever 2