Developer Tools (free)

7-zip

.net debugger

IE developer toolbar

viewstate decoder
http://www.pluralsight.com/tools.aspx

Application Verifier
http://www.microsoft.com/downloads/details.aspx?FamilyID=bd02c19c-1250-433c-8c1b-2619bd93b3a2&DisplayLang=en

Ethereal
http://www.ethereal.com/download.html