Yes, Halo goes up in the list, but they have made a Beta version for a Doom III co-op MOD, and I would friggin love that.
Also, try Doom 2 on Nightmare with a buddy, and Marathon on the hardest difficulty mode together. A friend of mine did that with me, and Nightmare on Doom 2 is a real *****, but we got far before I had to leave. Cheers! (PS, we were using the enhanced Doom 2, with really neat graphical updates and new client for LAN play)
Serious Sam II is hella-fun on Serious mode with Extra enemies. Try that.
Lets see... I cant think of anything else.