Network
WireShark (Cross Platform)
Fiddler 2 (Win32)
(Inject into SSL Requests)
Browers Instrumenting
Firebug (Firefox)
FireQuery
FireRainbow
Html Validator (FireFox)
Selenium IDE (FireFox)
LeetKey (FireFox, mucking with URLEncoding/Base64 etc)
Internet Explore Developer Tools (IE of course)
Chrome's built in tools
Safari's built in tools