HTTP Scoop is an HTTP protocol analyzer (sometimes called an HTTP Sniffer) for Mac OS X. It passively captures HTTP conversations between any HTTP client and server and presents their details in real-time. You can view all different aspects of the conversation: request URLs and parameters, request/response headers and POST data. You can also view the returned files as text (syntax highlighted for HTML and XML) or in hex or image format, as well as saving them to disk.
Version 1.4 adds slicker updates, response timing and easier navigation.