AssaultCube

AssaultCube is a open-source freeware Counter-Strike clone made with the Cube engine. As in that hugely popular game, it is a team-based multiplayer first person shooter (with bot support for the single-player mode). It comes with a map editor integrated, with support for cooperative editing with other players.

The game has the standard gameplay of the genre, where the basic objective is to kill the enemy with the available weapons. The setting is "realistic", within the limitations of the engine. The player starts carrying a knife, but can pick up pistols, assault rifles, sniper rifles, submachine guns, shotguns and grenades.

There are nine game modes (not counting the bot variations): deathmatch and team deathmatch, one shot one kill (where the players only carry sniper rifles) and its team variation, last swiss standing (where the players carry only knives and grenades), survivor and team survivor (like deathmatch, but the killed players are not respawned until the next round), pistol frenzy (players only carry pistols, knives and grenades) and capture the flag.

Due to its low hardware requirements, the game can run on old machines and use 56K modems for networking.