M.U.G.E.N

K-man's Mugen
Provides character information and artwork.
http://k-man2001.tripod.com/KmansMugen/

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

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

NekoMiMi: InfinityCat
Wiki article on InfinityCat, a M.U.G.E.N clone.
http://nekomimi.cafe.coocan.jp/wiki/?InfinityCat

RandomSelect
Covers the game engine with news, downloads, tools, and add-ons.
http://randomselect.i-xcell.com

Simple MUGEN 2004
Contains information and projects.
http://www.geocities.com/simple_mugen_2004/index.html

Street Fighter II NES
A Street Fighter II conversion for M.U.G.E.N.
http://sf2nes.smeenet.org

Wikipedia: M.U.G.E.N
Open-content encyclopedia article covering the origin of the game engine, history, description of gameplay, development, and controversies.
http://en.wikipedia.org/wiki/M.U.G.E.N