I wouldn't mind some Doom 1/2 deathmatch or coop. Perhaps some SNES games, as well. SNES9x has an online feature, and I can easily obtain most ROMs.
Hmmm... some of these other games sound fun. The older the better, because they're easier to... obtain. I can acquire almost anything, but stuff with copy protection makes it a little more difficult. Maybe we could play some Quake 2 Coop?
While I'm at it, anybody up for some UFO2000 multiplayer?
the capcom D&D game is pretty cool.
OH GOD, Shadow of Mystara is SO GOOD! I vote this! WinKawaks has an online play feature, as well. I can put the rom up if anybody needs it.
I also vote Freelancer. Good fun game.
As for Doom Source ports, JDoom and Doom Legacy aren't bad.
LAST EDIT: Actually, this is just a suggestion, but we could play some Aliens vs. Predator 2. A good game, and it's fun to play with other people. It's a Lithtech engine thing, sadly, and there's no coop, but the multiplayer is fun, regardless.