Nikhil Kothari has developed a browser extension for Internet Explorer that shows debug/trace information useful for ASP.Net developers.  The Plugin also displays the decrypted content of the application's viewstate.  For security purposes the app only runs for sites being developed on the local machine -- localhost.  You can find out more about the app and download it from his blog at http://www.nikhilk.net/Entry.aspx?id=63.

[Via Andrew Connell]