Now, i'm not really a fan of Newgrounds, in fact i think it's a massive cesspool of failed creations, but there's always that rare chance that you find a diamond if you sift through some turds.
I believe this is one of those of diamonds.
Super Mario Crossover is really exactly what it says, a crossover in Super Mario bros. It's the first game, but instead of just playing as mario, you get to pick from 5 other famous characters.
And all characters that are present in this game have the same functionality in the game they are in. For example Mega Man can slide, Simon Belmont can throw axes, etc.
Of course, Rather than getting powers where you'd normally expect, like candles, and robot masters, all characters get their respective powerups through the usual mushrooms and fireflowers.
Keep an eye out for version 1.1 coming out soon, it'll bring in Ryu Hayabusa from the Ninja Gaiden Series: http://www.youtube.com/watch?v=Zdr95SSgm2g
Again, since it's Newgrounds, probably going to throw some pop-up ads, nab yourself Adblock and you'll be set.