Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching information).
IE NetRenderer allows you to check how a website is rendered by Internet Explorer 9, 8, 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 - it's free!
Browserscope is a community-driven project for profiling web browsers. The goals are to foster innovation by tracking browser functionality and to be a resource for web developers.