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.
irrEdit is a free realtime 3D world editor and radiosity lightmap generator. It can be used as irrlicht scene graph editor, world editor, particle system designer, meshviewer and more. It simply stores scenes as .irr files which is the native format of the Irrlicht engine.