libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Mac Mouse Fix - A simple way to make your 3rd party mouse better.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Smooze - Smooze animates your scroll and adds functionality to your non-Apple mouse (scroll-wheel mouse).
Pygame - Pygame is a set of Python modules designed for writing games.
Smart Scroll - Smart Scroll makes scrolling smoother, faster and more comfortable: