GameSpy: This includes server addresses to Quake, QuakeII, and Hexen II. Easy to use and a quick way to find games. They include an overview of how many players are on each site and your ping rate.

Kali: This will give addresses for lots of games like; Descent, Diablo, Age of Empires, Outlaws, Armored Fist, Duke Nukem 3D, Forsaken, Mechwarrior 2, Starcraft, and many others.

The Zone: This site is for Microsoft's Internet Explorer but they now have made it available for Netscape 4.0 and above. They include games like; Fighter Ace, Quake II, Outlaws, X-Wing vs. TIE Fighter, CART Precision racing, Golf98, and Monster Truck Maddness 1 & 2, just to name a few.