Well, here's something fun for all of you oldschool gamers out there.

http://www.doomsdayhq.com/

Apparently, this group made some sort of overlay graphics engine for Doom/Ultimate Doom, Doom 2, and Final Doom. They also did Heretic and Hexen. You install the original games, then this "Doomsday" program. Not only does it handle launching and modding all of the games, it also gives them all huge graphics overhauls, adding richer texture details, dynamic lighting and sound, and even model packs to turn all of the enemies, weapons, and items into full 3D models. Furthermore, it adds stuff like the option to mouselook and turn off auto-aim, and even jump. That doesn't sound like much today, but for those who have played the original Dooms... that's pretty crazy. There's even a soundtrack pack for the Doom games that updates all of the music.

So, if you're ever in the mood for some oldschool PC gaming action with an updated twist, give this a try. It's no Doom 3, but I'd say it's worth it, if not for the simple reason that the oldest FPS are probably some of the most fast-paced FPS there are. Just turn on auto-run and play in Nightmare mode. You'll be dancing like Bojangles. I swear, that marine could tear-ass across a football field in five seconds...

They ported the Doom engine to windows, and added fancy stuffs, like lobby based TCP/IP multiplayer. Probably one of the better source ports (there are hundreds), but I still prefer zDoom for singleplayer, though it's not quite as advanced and has horrible netcode.
Doom/Hexen coop = yes.