Irrlicht looks like a very cool project and, despite being at version 0.10.0, appears to be stable and in use by quite a number of applications. I especially like the focus on ease of use, the great (easy) tutorials, and that fact that .NET is supported.

Here's the description from the web site:

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, bindings for java, perl, ruby, basic, python, lua, and so on. And best of all: It's completely free.