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.
TacitPixel is a scalable C++ framework for building application prototypes for interactive collaborative VR/AR applications. It provides a number of features that make prototyping fast and portable.
TacitPixel is the successor of the TAP and Twisted Pair engines.