One of the most powerful and feature-rich traffic monitoring tools available on the Internet is Fiddler by Eric Lawrence. Unfortunately, Mac users won't be able to use it as the official developer has not yet released Fiddler for Mac. If you're looking for an alternative, you're just in the right place. With this list with Fiddler for Mac substitutes, you can find similar tools that will allow you to monitor the HTTP traffic between your Mac and the Internet, or even test web applications.
Feel free to add any alternative to Fiddler for Mac that you know of.
Suggest Alternatives
They also say that Wireshark is a nice alternative http://macdownload.informer.com/wireshark/