Many animation effects are simply gratuitous eye candy -- however, there are situations where animations effectively communicate to the user what's going on. What are some of your favorite uses for animations, and what specific animation type would you use?
E.g.: Animate items downwards when a new item is inserted into a list
There is a very nice paper by Ben Bederson and Angela Boltman in which they evaluate the impact of animation on user’s ability to build a mental map of the information in the space: Does Animation Help Users Build Mental Maps of Spatial Information?