toolgawd: Quake 2: Capture the Flag - Railguns Only.
I think I wasted a good 2 years of my life playing this on HEAT.net. This was the pinnacle of multiplayer shooters and will always be in my opinion.
If you're looking for something else...I can't really think of anything else. Everyone has suggested all the FPS' I used to play.
Makes me more happy that I found my copy of Quake 2! :D
Theta_Sigma: Hmm, I found a copy of Doom 2 but it's only the shareware edition and it's MP is non existent for some reason. :(
F4LL0UT: I'm quite confident that you can use any source port such as ZDoom or even the official (if old) Doom 95 for LAN play, even if you only have the shareware episodes. I remember putting my original DOOM2.WAD file into the folder of the shareware version of Doom 95 and the LAN play worked without any issues. I think it should also work with any shareware version's WAD files.
Awesome, that's good to hear, I'll look for ZDoom in a bit. I actually have FreeDOOM on my laptop. THANK YOU LINUX REPOSITORIES! ^_^