http://www.newgrounds.com/portal/view/498969 - Super Mario 63! Basically, it's a 2D version of Super Mario 64, with the FLUDD pack from Sunshine. It also comes with a nice level editor, and you can share and play other people's levels.
http://www.newgrounds.com/portal/view/301341 - The Fancy Pants Adventure! This platformer's a little bit like Sonic, what with its insane speed and such. It focuses more on physics and momentum, though, and it's a lot of fun. The sequel is even better.