The Irrlicht Engine is an open source high performance realtime 3D engine
written in C++.  It is completely cross-platform, using D3D, OpenGL and
its own software renderer, and has all of the state-of-the-art features
which can be found in commercial 3d engines.

It has a huge active community, and there are lots of games in development that
use the engine.  You can find enhancements for Irrlicht all over the web, like
alternative terrain renderers, portal renderers, world layers, tutorials,
editors, bindings for java, perl, ruby, python, and so on.