Kong!

A multiplayer 3D top-down shooter with a thirst for blood

The rules are simple, pick up as many health/armor packs and weapons as you can before blowing your enemies into smithereens. The game currently features three game modes including deathmatch, team deathmatch and capture the flag with plans to introduce a number of new game modes. Computer controlled bots are provided as target practise but the real fun comes from playing in multiplayer over a local area network or the internet.

Cube

by Wouter van Oortmerssen aka Aardappel

A very nice first person shooter

A very nice open source multiplayer and singleplayer first person shooter game

Hover Tank 3D

Hover Tank 3D is a tank vs. tank game where you duel the enemy through various modes of play and numerous arenas.

Cross Fire

Cross Fire is an online military first-person shooter for the PC, developed by SmileGate and published by G4BOX. Players assume the role of either a Black List or Global Risk operative, and then compete in modes including Team Death Match, the exclusive stealth-action ghost mode and the brand new Mutation Mode. Cross Fire also includes a persistent military ranking system, in-game friends lists, a clan formation system, daily missions and deep character customization options. Cross Fire will always be free-to-download-and-play.

Halo Combat Evolved Custom Edition

Halo Custom Edition is a multiplayer-only stand alone version of Bungie's sci-fi shooter. Halo CE includes several new multiplayer features and options, the most notable of which is the ability to play user-created content. This add-on is unsupported and requires a Halo CD and Key to install.