I love my oldskool games!
I've got alot of Windows ports to alot of my older games.
DooM I and II - This is great if you have the Doomsday Engine Windows Port, makes it look SO much better. Also has TCP LAN/Internet Support, and its awesome

Duke Nukem 3D - Someone actually made a good Windows Port to it, I need to find it again though. Has Internet/Network Support too.
Quake 1 DM, and Team Fortress! (Hell yeah!) - Again, I use the fuhQuake Engine for that added 3D goodness

Great for Single/Multiplayer
Quake II - Great fun, mates and myself sometimes play it still every so often.
There's plenty of other games, but those are the main four I enjoy.