Urban Terror!

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.

Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself. Xonotic is a direct successor of Nexuiz with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.

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

Unvanquished is a free, open-source first-person shooter combining real-time strategy elements with a futuristic, sci-fi setting. It is available for Windows, Linux, and Mac OS X.

A free-and-open-source first-person-shooter aimed at improving the classic game, AssaultCube!

Warframe situates players as members of the Tenno race, newly awoken after years of cryo-sleep into a solar system at war. Reborn into a corrupt era, the Tenno are sought by the oppressive Grineer Empire for annihilation. Warframe armor is the key to overthrowing the Grineer by providing players with unique offensive and defensive powers to explore, upgrade and master during purpose-driven radical raids.

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.