http://www.newgrounds.com/portal/view/534416
Someone created a flash game that's a near exact copy of SMB 1 for the NES, but with selectable characters from other NES games such as Megaman, Metroid, Castlevania, Zelda, and Contra.
Each character controls just like they do in the game they come from.
Oh and you can also play as Mario.
Apart from some minor level modifications and backwards scrolling, and the ability to save your progress it's exactly like SMB 1. (Requires the newest Flash player)