A composition and general post production video touch-up program formerly produced by Nothing Real and currently produced by Apple. Shake is one of the most widely used composition programs today, available for Mac OS X (the newest platform, which sort of followed after Apple bought the company), Linux, IRIX and Windows NT.
The program is able to do composition of images and keying, color/channel correction and applying filters, various image transformations and warping, and painting and rotoscoping. It also supports all sorts of APIs and plug-in development.
As said, Shake is widely used by the Big Boys - the chances are that many cool special effects you see in The Next Big Movie were added to it with using Shake, or otherwise got messed up with using some of the gigantic arsenal of tools it has. The people who use it praise it endlessly, mostly thanking its amazing flexibility, speed and stability.
Shake is apparently to Final Cut Pro what After Effects is to Premiere. Of course, the magnitude of these products is entirely different...
Home page: http://www.apple.com/shake/