The Irrlicht Engine is an open source high performance realtime
3D engine written and usable in C++ and also available for .NET
languages. 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.

We've got a huge active community, and there are lots of projects
in development that use the engine. You can find enhancements for
Irrlicht all over the web, like alternative terrain renderers,
portal renderers, exporters, world layers, tutorials, editors,
language bindings for java, perl, ruby, basic, python, lua, and so
on. And best of all: It's completely free.