Recently, when I was developing the Post Admin Shortcuts WordPress plugin, I needed a progress indicator to show the user that something was happening in the background. So I Googled a bit and found quite a few sites where you can download or generate AJAX loading indicators. Here are the top 5 sites that I found:
(In no order of preference)
Some really cool and unique loaders can be found here. You can also create variations based on each.
Advanced options allow for almost unlimited variations.
Web Script Lab
Super simple and to the point. Generate a loader in seconds.
Some of the best looking 3D loaders on the web can be found on Preloaders.net
One of the original ajax load generator sites. And still one of the best.
Do you know of any good generators that I have not mentioned?