From MacPowerUser, iDraw is a powerful, yet easy to use, illustration and animation program for Mac OS X that has many of the features of high-end design programs, such as smooth anti-aliased rendered paths, different fill types including semi-transparent colors and gradients, object transformations such as tapers and arcs, text shape manipulation, and much more.
With the ability to create still illustrations as well as frame-based animations that can be output to SWF files or GIF animations, iDraw is perfect for anything from detailed designs to simple web animations.
More info and download link here.