Before I start, the Atlas Control Toolkit just recieved another Update, which includes an Animation Library. Check it out!
Scott Guthrie posted an amazing post on AJAX/Atlas Activity Image Animations
Indicators are a very useful tool that help users understand what is happening with a website. Too often a user will double, tripple, 23483.2 click a button if they see nothing happening.
Displaying something like: Loading Please Wait...
Will put users at ease knowing something "is" happening. Even if it's just an image swirling about, it will still delay them for about 5-10 seconds before they start smashing the "Submit" button again.
Image from: http://www.napyfab.com/ajax-indicators/