3D Platform Game

Hi, I would like to implement a small 3D engine. I am thinking of a cute character that jumps around in a 3D world (platforms etc) and collects shiny things. Nothing fancy. Now my Direct3D skills would probably suffice to do this. But rather than teaching myself how to construct and manage a 3D world from scratch, I''d like to take a look at a well-programmed example. I am sure something like this can be found on the net somewhere. Who has an idea on where to start? -seb