The Marathon Series (by Bungie)
http://source.bungie.org
This game is essentially the predecessor to Halo (there is even a couple of achievements in Halo games with marathon-related 'titles'), and is definitely worth checking out, it was released into the free domain by Bungie quite some time ago, just download Aleph One + the Marathon pack that you are interested in and you're all set.
If you want the key storyline, play Marathon, Marathon 2 and Marathon Infinity (in that order), if you want something a little different then try Marathon Evil or Tempus Irae (both use modified Physics engines and new/modified weapons)
EDIT: Also regardless of if you're new to Marathon, or a returning player just for nostalgia, check out Marathon: Eternal X here [http://eternal.bungie.org/] It is one of, if not THE biggest fan-made Marathon project, containing 52 levels, so it will last you a while.