FreeWRL is an Open Source, cross platform VRML2 and X3D compliant browser, with script, SAI and EAI support. Platforms supported: Linux and other Unix-style platforms; Mac OS/X; Windows. Support for mobile platforms (iOS, Android, QNX) is under development.
The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++. It is completely cross-platform, using D3D, OpenGL and its own software renderers.