AssaultCube a FREE, multiplayer, first-person shooter

AssaultCube, formerly ActionCube, is a free and open source software first-person shooter video game, based on the Cube engine.
It takes place in realistic environments, with fast, arcade gameplay. Although the main focus of  AssaultCube is multiplayer online gaming, a single-player mode consists of computer-controlled bots.
AssaultCube utilises efficient bandwidth usage, allowing the game to be run with connection speeds as low as 56kbit/s. It can run on older computer hardware as well.
AssaultCube is available for free on Microsoft Windows, OS X, and Linux. The game engine is free software, however parts of the accompanying game media, such as the graphics, are released under non-free licenses.
AssaultCube was designed to be more realistic and team-oriented than Cube while keeping Cube’s distinctive fast-paced gameplay. In comparison to other games, AssaultCube is slower than Quake but faster than Counter-Strike.
There are two different teams in AssaultCube, called the “Cubers Liberations Army” (CLA) and the “Rabid Viper Special Forces” (RVSF).” source: wikipedia

You can download the game client from the project webpage or install it directly from your distribution repository through package manager.