OpenArena!

Overwatch is a team-based shooter where heroes do battle in a world of conflict.

Red Eclipse is a fun-filled new take on the first person arena shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments.

Warsow is a completely free fast-paced first-person shooter (FPS) for Windows, Linux and Mac OS X, set in a futuristic cartoonish world where rocketlauncher-wielding pigs and lasergun-carrying cyberpunks roam the streets.

Cube 2: Sauerbraten is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.

TrueCombat: Elite is a FREE modification for the Enemy Territory game (also free). It is a realistic modern day FPS featuring state of the art models and textures.

Freedoom is a free first-person shooter under continuous development. The Freedoom project aims to create a complete free content game based on the Doom engine. Combined with the engine, Freedoom is also compatible with game modifications made for the original Doom games, made by Doom fans and artists over the decades.

World of Padman is a incredibly meticulously designed and multicolored freeware fun-shooter for young and young-feeling people. You make your way through lilliput maps equiped with plastic guns.With 5 game types, 11 player models (even with diverent skins) and over 20 maps in different variations, WoP 1.5 is appareled with a ton of features and thus one of the biggest freeware shooters.

Team-based multiplayer arena shooter made by the Borderland developers.

Blacklight Retribution is a fast paced, adrenaline pumping free to play first person shooter (FPS) based in a futuristic urban warzone. Blacklight Retribution provides FPS enthusiasts with a massive arsenal of weaponry and gear ranging from bleeding edge side arms to massive mechanized hardsuits.

The website linked is not real page, just moddb page. Wikipedia says all official sites have been closed.This program is now available online for free at abandonia.com.