Most useful animation in web or desktop application

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

From stackoverflow

• From enjoy3d.com

  

  Press your mouse button

  and move to look around.

• One example I can think of is the animation used by operating systems when you minimize a window.

  Both Microsoft Windows and Apple OS X animate the window going down to the taskbar (or the Dock in OS X) to show the user where the window went. Otherwise novice users that hit minimize by accident might have trouble getting the window back.

  I don't use linux, but I'm pretty sure it does the same. I'm not being discriminative =)

• I really like Google Chrome's use when a file is being downloaded. It's hard to describe, but, it's a circle that fills like a pie chart as the download progresses, and the circle is overlaid with the icon for the file you're downloading. Very slick.

• Progress bars are nice for things that take more than a moment or two, but only when they are accurate. An inaccurate progress bar is worse than none, in my opinion.

• I believe that all visual changes should not be swift. Be it status notification, window maximized/minimized, or data deleted/added. I cannot find a reference, but usually it is recommended that all animations should not be around 1-2 seconds, matching human's response time.

  My favorite uses of animation is not in a commercial software (though Apple is good at this) but a research paper called Phosphor which I consider one of the great UI ideas that have not yet implemented into major operating systems.

• AJAX loading gifs - you've got to have an indicator that you definitely registered an event and you're doing something about it

• 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?