A mac programming competition (typically entries are in Cocoa), see http://ironcoder.org
After each competition the source code is available for all entries - and is a chance to see tricks and techniques used throughout the community.
The judge provides an API and a Theme - then you have two days to code a solution.
Previous competitions have been:
*I - (March 3rd 2006) Accessibility API and Mardi Gras *II - iTunes visualization and Emily Dickinson *III - Quartz framework and Time *IV - CoreImage and Space *V - (March 30th 2007) ScreenSaver framework and Life