To my unpracticed eye, the Irrlicht 3-D engine looks interesting.
Several years ago, I was going to take the summer off and write a 3-D game engine (or at least the beginnings of one). I got a month or so into the project and wound up doing more fun things with the summer. About the only thing I came away with were some C++ wrappers for Direct3D. [It sure was a refreshing change from hacking a Java server…]