3D .NET engine for Windows, Linux, Mac
Cocos2d for iOS and OS X, built using Objective-C
A Flash Molehill (Stage3D) GPU accelerated 2D game engine
Simple workflow engine written in Python to manage program executions.
The Flixel Power Tools are a set of plug-ins for Flixel 2.5+
BxEngine For Marmalade And QT
Cross platform 2D game creation framework with Python bindings
Formerly CraveJs, it's for creating JavaScript applications and games.
C++ SDL/Box2d Programming
A generic framework for the generation of game levels.