I was hoping this would be the belly-sliding game that I had on Mac OS X circa 2001. Someone made it with whatever new-to-MacOS tech to show off what could be done that wasn't possible on Mac OS 9 and I downloaded it from somewhere.

Was a fun third-person 3d racing game of some sort. Tux raced down a hill covered in snow on his front and you could steer around trees and rocks and such. My memory is probably getting the details wrong.

Edit: haunter had the answer. It was a port of TuxRacer.