Fiddler Web Debugger - Freeware HTTP(S) debugging tool
MarineHero
: "Best tool to hack/test/enhance IE... it supports plugins and has a rich object model... great for debugging, testing and diagnosing strange problems. For FireFox I use FireBug... but Fiddler is so much more extensible, I have used it in the past to record web sessions and generate VSTS load test BOTs right down to source code."