Alien Vs. Predator Lair
Boulevard 64
A large gaming site -there are articles about scrolling beat 'em ups, but there is also a lot of other content which is well worth checking out.
Capcom's official site.
Capcom Wiki
Death Adder's Castle
Double Dragon Dojo
Final Fight Double Impact
The official site for Final Fight Double Impact.
Streets of Rage Online
The Cutting Room Floor
This site has a look at unused content in various games, and there are a few Final Fight games covered there.