I only just came across this great post at watchingapple.com from last year that describes many of the key animations in OSX (pre-Leopard).
For many years animation had a bad reputation amongst UI designers, mostly because of its "mis-use" in banners ads on the Web. Fact is, our eyes and brains are hard-coded to notice movement in our peripheral vision (it's so that we see that saber-toothed tiger in the bushes). Banner ad makers know this and capitalise on it.
However, this power to attract attention can be used for good, as well as evil, and we are seeing more and more use of appropriate animations and transitions to draw people's attention to activity in the periphery, and also to help users understand:
- The cause-and-effect relationships between user actions and system responses.
- The 'journey' the user is taking through your application and data.
If you need to design for animation, bookmark the watchingapple.com article - it will be a nice little ready-reference of different animation ideas. As you browse through watchingapple.com you'll find lots of other references to animation in OSX and other Mac products too.