CND (Chip N Dales MMORPG) is a sidescrolling arcade game that you can play with your friends over the internet, completely free. Transform into your favorite retro-characters including Mario, Link, Sonic and Pikachu as you adventure through RetroWorld, defeating bosses, collecting trophies, unlocking characters, finding unique items, and other things. Decorate and develop your character and your in-game house and show them off to your friends. Defeat other players in sidescrolling arcade-like PvP action!

Altough I have to say the game have been glitchy recently, it's a game you have to try. Here's the link:

http://cnd-online.net/

Here's the Official Picture.