Fiddler Web Debugger
Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data.
IETester - IE 5.5/6/7/8 in tabs
IE NetRenderer allows you to check how a website is rendered by Internet Explorer 7, 6 or 5.5, as seen from a high speed datacenter located in Germany. Just type in a URL in the field above and try it out.
Firebug - Web Development Evolved
DebugBar - IE extension for web developer