For a Software Architecture for User Interfaces class, I implemented a pie menu (also known as a radial menu) for Mac OS X. I did not know Cocoa (or Objective-C) very well ahead of time, so I learned Cocoa and put together this pie menu in the span of about two months.
I wrote up a final paper for the class detailing my experience: Cocoa Pie Menu Implementation [pdf].
Julian Missig