Multiplayer Doom: ZDoom
We all remember the original Doom, the game from id Software that started the FPS genre.
If you want to reminisce or simply have a totally new gaming experience, we recommend ZDoom.
ZDoom is classic Doom, and you can play it on any Windows platform from Win95-Windows Vista and even Linux!
ZDoom supports cooperative and deathmatch multiplayer online gaming modes! The protocol is UDP/IP.
More about ZDoom multiplayer can be found here: http://zdoom.org/wiki/Multiplayer
A Doom LAN party never sounded so fun!