Well, there is always arguably the best FPS ever made. DOOM. The original version wasn\'t online, but you could dial up to a friends house and frag him till he cried. Thankfully, ID released the source code to Doom/DoomII and fans have been able to create updates and patches, which add features like: True transperancies, High Res, Blood splatters, rocket trails, a Console ala Quake, Jumping/vertical aiming, and of course... true online play. Speaking of which, there are still a few groups of Quakers and Quake IIers out there who still play regularly.
Master of Orion II is an excellent space strategy game, and online through the WoN network I believe. Dunno if you can still connect with it. I reccomend downloading/buying both of the above games, even if you can\'t get em to work online simply because they are classics in their genres and shouldn\'t be missed.