Ok, so my little del.icio.us app (click link to read about how I added Ajax functionality to a simple Rails app) is pretty cool, but it was missing one big thing. When the user clicks the "Get Results" link she has no idea that the page is communicating with the server. While the communication doesn't go on forever, an impatient user might click the link multiple times.
So, we need a way of letting the user know that something is happening and that they ought to wait a bit. Today, the standard seems to be an animated spinning icon: