I Read very nice post that has written by Matt Berseth regarding debugging asp.net AJAX applications. In his post he examples  a tool that has been written by the YUI project team has developed and compares it to the Trace Console AjaxControlToolkit Control that belongs to Microsoft.

If you are a asp.net developer that uses AJAX extensions or even not, this post can help you debugging AJAX events and actions.

You can read it here.