http://www.newgrounds.com/portal/view/534416
Play through the entirety of Super Mario BROS. 1 with a wide selection of characters to play as 😊
Super Mario BROS. Crossover.
http://armageddongames.net/showthread.php?45447-Super-Crossover-Bros.
It is a pretty fun game though, so I can see why you would want to make a thread about it. I still say Samus is my favorite character...
ffff yeah, I saw it at its homepage first but couldn't remember it so copied the link from the thread at PureZC.
I've played it and thought it was brilliantly done, though I'm not a fan of the 2D Mario games, so I got bored of it rather quickly.
A forthcoming update will have Ryu Hayabusa from the Ninja Gaiden series. Video (make sure you watch to the end!).