Wolfenstein (series)!

Team Fortress 2 (TF2) is the sequel to the game that put class-based, multiplayer team warfare on the map. One of the most popular online action games of all time, TF2 delivers constant free updates including new game modes, maps, equipment and, most importantly, hats.

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

Urban Terror started as a realism based "total conversion" mod for Quake III Arena. Nowadays, Urban Terror is played using the free, stand-alone, ioquake3 engine modified, which doesn't require Quake III Arena, in addition to being an add-on to id Software's Quake III Arena.

Squad is a 50 vs 50 multiplayer first-person shooter that aims to capture combat realism through communication and teamplay, emphasizing both strong squad cohesion mechanics as well as larger scale coordination, tactics and planning. It features large open maps, vehicle-based combined arms gameplay, and player-constructed bases to create a heart-thumping, visceral gaming experience which mixes organized tactical multi-squad planning with split second decision-making in real-world scale firefights. »

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.

Wolfenstein: Enemy Territory is a free multiplayer first-person shooter. Set during World War II and heavily focused on team work, it was initially released in 2003 by Splash Damage and id Software. More than a decade after its release, after spending countless hours of escorting tanks and trucks, stealing gold and radar parts and transmitting confidential documents, it was time to dust off the game which consistently refused to go down.

Dirty Bomb is a first person shooter with focus on teamplay, movement, variance in loadouts and objectives. It features multiple game modes.

One of the most popular online action games of all time, Team Fortress Classic features over nine character classes -- from Medic to Spy to Demolition Man -- enlisted in a unique style of online team warfare. Each character class possesses unique weapons, items, and abilities, as teams compete online in a variety of game play modes.