OpenSceneGraph!

OGRE (Object-Oriented Graphics Rendering Engine) is a scene-oriented, flexible 3D engine written in C++ designed to make it easier and more intuitive for developers to produce applications utilizing hardware-accelerated 3D graphics. Since 2001, OGRE has grown to become one of the most popular open-source graphics rendering engines, and has been used in a large number of production projects, in such diverse areas as games, simulators, educational software, interactive art, scientific visualization, and others. »

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 renderers, 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. »