2D/3D Engines

There's one place on the Web that's the focal point of all 3D engine development. It's called The 3D Engine List and contains 3D engines of varying levels of technology. The amazing thing is that many of the authors will let you use their engines for free! Here's the address:

http://cg.cs.tu-berlin.de/~ki/engines.html

In addition, here are some links to some specific 2D/3D engines that rock!

Genesis 3D Engine	http://www.genesis3d.com
SciTech MGL	http://www.scitechsoft.com
Crystal Space	http://crystal.linuxgames.com/