http://trilogyrelease.bungie.org/

Bungie allows anyone to freely download a copy of any series of Marathon. It is legal.
__________________________________________________ ___________________

http://marathon.bungie.org/maraserialgen/

This link generates serial codes for Marathon games, for free. It was recently released by Bungie. Due to that it is hard to find a serial code for any of the Marathon games. This is also legal.