Adrenaline is a freeware multiplayer top-down shooter created with the Game Maker engine, featuring a wide variety of weapons to use, four different game modes and support for user-created maps in addition to the official maps that comes with the download. The latest version of the game (1.0.0, released on January 1st 2010) includes plenty of graphical settings that can be tweaked to accomodate slower computers, a single-player mode with bots, and even stat tracking for registered Reflect users.

Besides your standard ranged weapon, you can also swing a knife for melee attacks or throw grenades to hurt other players. The frequent disconnections and lags could disrupt your enjoyment of it, but if you are looking for a multiplayer deathmatch game that is free to play then Adrenaline might fit the bill nicely. (source, controls)