Game engine supporting many 3D/2D formats and graphic effects
Orx: Portable Game Engine
Vivid3D is an open-source 3D engine with powerful 2D UI.
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Modern 3D engine and IDE written using C# and C++.
Multidocument automation by templates - for sphinx, mkdocs, epydoc ...
2D ARPG Engine
evolution for vegastrike SpaceSim/Combat/Trade game
Pygrid is an open-source and cross-platform pygame/python 2D engine.
Open Source Virtual World and MMOG Development Platform
A powerful cross-platform and open-source 3-D rendering engine.
A generic C++ MUD server featuring a strategy-style hexagon map