"A shooting game very similar to Quake III and free"
OpenArena is a free distribution first-person action game. It uses the same graphic engine of Quake III, so they are very similar.
Choose between two different ways to play Open Arena, you can choose to fight against the Artificial Intelligence of the game, you alone against the machine or play against other participants over the Internet in its online multiplayer mode.
It's about getting into the different rooms and killing all the enemies you find. As simple and complicated as that. Aim, shoot and finish them all before any of them finish you off.