FlightGear Flight Simulator: free open-source multiplatform flight sim
3D Engine c++
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
3D .NET engine for Windows, Linux, Mac
action/combat flight sim
A cross-platform game framework based on OpenGL and GObject.
Multi-Language Wrapper Collection for the Irrlicht Engine
Volume Rendering library in Java
1st 3D game engine built in C# for .NET, Visual3D Engine's predecessor