CGSPrivate is a super cool header which allows Mac developers to use nice animations on their apps. There are many animations, but the most popular one is the cube. On the Mac OS X, when you switch the user usingthe fast user switching, the desktop rotates and the other user’s desktop appears. You can use that and many other animations on an ordinary NSWindow with a few lines of code.
CGSPrivate uses private APIs, which means it can stop working anytime as Apple may update Mac OS X and remove the support for the animations with no prior notice. Mostly likely it’s not going to happen any time soon, as CGSPrivate has been here for a couple years(the first version of CGSPrivate was wrote back in 2003, I believe). It also can’t be used on apps you pretend to sell on the Mac App Store. Apple doesn’t like apps that use private APIs.