Unreal Tournament
Multiplayer-centered first-person shooter released in 1999 by Epic.
To get the game working properly, all power saving measures such as AMD's Cool'n'Quiet/PowerNow! or Intel's SpeedStep need to be disabled, because the initial low clocks when starting the game will throw it off, causing erratic fluctuating framerates. This can be done by setting the power scheme in the Control Panel to something like "Home/Office Desk" or "Always On". Also, cwdohnal's fixed OpenGL renderer called UTGLR is recommended when playing the game on hardware other than 3dfx or S3 Savage.
When using S3TC textures online, one may get the "sliding players" bug. To fix this, the appropriate textures need to be patched. See the linked guide for details.
For online multiplayer, today Windows 2000 or higher is advised, because most of the servers use a newer ACE (AntiCheatEngine) version that dropped support for Win9x.