Seven things in Halo that are the same (or better) in Marathon.
This podcast uses:
Aleph One, free and open source versions of Marathon for Windows, Mac and Linux at http://source.bungie.org
Garry's Mod, available from Steam.
Mark V[B] Spartan model for Garry's Mod by 017.
Halo M6G and SMG weapons by Residual Kat.
SCars Slim by Sakarias88, Scars Halo by Lucky9Two, and Russian cars by Denz.
Blood Fury's cinema tools.
Greenscreen Material by DasMatze.