Beat Hazard
Audiosurf
FEAR
Metro 2033

big rigs over the road racing

Baldur's Gate, Icewind Dale, NWN for a bit of older school RPG goodness.

Diablo II if you like very addictive relatively deep and engaging hack and slash.

Modded Morrowind / Oblivion would be very high on my list as well. The talented folks in the modding community offer the opportunity to further enhance these (already fantastic) games to suit your personal preferences.

Limbo of the Lost

Dark Messiah: Might and Magic.
Magicka.
Team Fortress 2

I don't know if the Half-life series are PC-only, but if you don't have those games I would highly recomend them.

Of course. And since it has bits and pieces plagiarized borrowed from almost every major title of the last decade, all the better!