Multi-platform 2D and 3D game engine
... the open-source glide computer
GLScene Engine for Delphi & C++Builder VCL Windows 32/64
Orx: Portable Game Engine
Open-DIS implements the IEEE DIS protocol in multiple languages.
3D .NET engine for Windows, Linux, Mac
A cross-platform OpenGL based lightweight scenegraph library
Cross-platform 2D game engine