Halo: Combat Evolved Custom Edition
Full user review
Gives much control over what your matches.
Great maps both single and mutiplayer from http://hce.halomaps.org/.
It's a pain 2 play it without the cd. You need a iso image and a program like magiciso or Dameon Tools.
If you have the cd GREAT, if not then with extra work...Great. I would recommend everyone get this free download!