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 ...
httpbin.org : A simple HTTP Request & Response Service.
Buildbot : Buildbot is an open-source framework for automating software build, test, and release processes.