I stumbled across a gem of a game today at kongregate.com, titled Abobo's Big Adventure.
This is the most successful NES crossover implementation I have ever seen. This game has everything. Epic cameos from every NES character imaginable, 10 or so different NES "worlds" to explore.(Worlds are basically a clone of the NES game it belongs to, with awesome differences) Awesome moves, Extreme yet very well used gore.
I cant rave about this game enough. I jsut beat it and it was unlike anything Ive played in years. The basic story is that you are Abobo(boss from Double Dragon) and someone has kidnapped you son, Aboboy. You strike out on a quest to rescue him from his captor. Along the way, you play through 10 NES Games with cameos from Hundreds of characters all along the way. The characters can be friendly, enemies, even bosses. Some of your favorite NES titles are covered here. Double Dragon, Zelda, Megaman, Punchout, Contra, Mario, and many others. Please check it out and let me know what you think.
http://www.kongregate.com/games/pestoforce/abobos-big-adventure