One of the Nielsen heuristics stands for
"Visibility of system status: The system should always keep users informed about what is going on, through appropriate feedback within reasonable time."
In modern web pages we've got a lot of asynchronous AJAX calls. To keep user informed what is really happening, we can simply show loading image while some calls are pending.
Small, smooth and easy to use describe this great library. Just download .js file, add few line to you css and Voilà - it works!