Any Marathon requires one important thing... LOTS of games! The initial list looks something like this:
Halo 2 - Lots and Lots of Halo... not just a little... huge amounts... in fact this might be all we play.
Links
Midtown Madness
Crimson Skies
Dead or Alive Ultimate
Rallisport Challenge 2
Others are still in the works... but that is a list of what we have direct access too... we would play some Mech Assault 2 if it had a matchmaking system. The lobbies in that game are torture.