If the thought of free frags makes your trigger finger twitch, take a look at AssaultCube.
This open-source first person shooter doesn't offer much in the way of snazzy graphics or major frills, and there's no storyline or plot. Just a selection of maps where you can hunt or be hunted by other players or computer-controlled bots. But the multi-platform AssaultCube can run on Windows, Linux or Macs, and its minimum system requirements are so low that just about any computer should be able to run it (Pentium III 500Mhz and nVidia GeForce 256 or equivalents, 128MB RAM). Plus, the entire game fits into a compact 40MB download.
You can join AssaultCube's online game community by hopping onto one of a number of online servers for multiplayer games, or you can battle against bots for some single player action. If you've ever played an FPS, the game will feel entirely familiar. You'll find machine guns, sniper rifles and grenades as weapons, along with health packs and other goodies scattered around a variety of maps. For guides and game patches, head to the Assault Cube wiki.
AssaultCube offers some solid free fun. If you're looking for a straightforward shoot-em-up, give it a try.