HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
Wireshark : Wireshark is the world's foremost network protocol analyzer. It lets you see what's happening on your network at a microscopic level. It is the de fac...
hurl.it : Hurl.it makes HTTP requests.
Choose the request method, customize headers and POST parameters, add basic authorization or OAuth credentials, and even ...
Mockbin : Mockbin allows you to generate custom endpoints to test, mock, and track HTTP requests & responses between libraries, sockets and APIs.
mitmproxy : mitmproxy is your swiss-army knife for debugging, testing, privacy measurements, and penetration testing. It can be used to intercept, inspect, modify...