Modders are people who deliberately modify games to their advantage or for fun. MonoxideC is a "famous" modder to the Halo community. He created Halo Map Tools, a tool for modifying Halo PC maps.
Also, a term when the word N00B has been overused.
Team 1: Hey, that modder is awesome! He helped us win that game!
Team 1: Yeah!
Team 2: FUCKING. MODDERS.