Inspector: View DOM Tree and modify tags attributes and css attributes on the fly to test your page
HTTP Inspector: View HTTP/S request to check cookies, GET and POST parameters, view server info
Javascript Inspector and Javascript Console: View javascript functions for easier debugging, see Javascript and AJAX code
HTML Validator: Validate HTML code to correct and optimize your code and html size of your page
And many more features: See page cookies, get pixel color on a page, make a page screenshot...
