Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching information).

Charles can act as a man-in-the-middle for HTTP/SSL communication, enabling you to debug the content of your HTTPS sessions.

Charles simulates modem speeds by effectively throttling your bandwidth and introducing latency, so that you can experience an entire website as a modem user might (bandwidth simulator).

Charles is especially useful for Adobe Flash developers as you can view the contents of LoadVariables, LoadMovie and XML loads. Charles also has native support for Flash Remoting (AMF0 and AMF3).

Charles is also useful for XML development in web browsers, such as AJAX (Asynchronous Javascript and XML) and XMLHTTP, as it enables you to see the actual XML that is flowing between the client and the server. Charles natively supports JSON, JSON-RPC and SOAP; displaying each in a simplified tree format for easy viewing and debugging.

You can download Charles Proxy here:

Windows – charles_setup.exe
Linux / Unix – charles.tar.gz
Mac OS X – charles_macosx.zip

Or read more here.