Among it’s features are XmlHttpRequest spy which allow you to see how websites are using XmlHttpRequest. I currently use LiveHTTPHeaders which supports standard requests as well as XmlHttpRequests. LiveHTTPHeaders is a really useful extension for debugging HTTP – for example it can help you work out where your cookie settings are wrong.
I also love the fact you can finally filter errors and hide CSS errors and internal errors and it’ll only show errors for the current page. Great stuff!