Examples of procedural geometry for Unity3D C#
2D Game Engine
A cross platform Lua based game engine
Build Cross-platform Applications
Asphyre is a cross-platform framework for Delphi and FreePascal.
3D .NET engine for Windows, Linux, Mac
Cross platform Game Creation Software
The classic intelligent role playing game
A cross-platform OpenGL based lightweight scenegraph library
Cross-platform 2D game engine