K-man's Mugen

Provides character information and artwork.
http://k-man2001.tripod.com/KmansMugen/

Simple MUGEN 2004

Contains information and projects.
http://www.geocities.com/simple_mugen_2004/index.html

Mugen Fury

Includes characters, stages, tutorials, and games.
http://www.mugenfury.com/

Mugenation

Offers news, characters, forums, and game information.
http://www.mugenation.com/