UT2K4 runs pretty well on a bevy of different machines, and I LAN with a few friends every now and then, even though I can max out UT3 now. Not multiplayer, but Deus Ex is excellent, and I recommend anyone with older hardware to try it. I think they may even have a multiplayer mod out for it though, and it runs in both OpenGL and DirectX, plus *gasp, S3 Chrome and software modes.