I'm a long time fan of Fiddler, an excellent tool for spying on and interfering with HTTP requests. Analyzing the headers and packets sent across the wire has been an invaluable source of information to me many times. Sadly Fiddler suffers from a drawback where it is unable to view localhost traffic by default. There are workarounds but these are not always practical. So recently I was interested to see HttpWatch.
HttpWatch integrates straight into Internet Explorer or FireFox and provides an analysis of browser traffic. In particular because it is simply using the browser it clearly sees all localhost traffic without any workarounds. Now, whether HttpWatch has more features and functionality than Fiddler is still open for debate but in this one issue at least HttpWatch is solving a problem for me. The "Basic" version of HttpWatch is free and the other versions you have to pay for (whereas there is only one version of Fiddler and it does everything and it is all free).