HTTP request and response data tool

Firebug for Firefox : Menu -> Net
Apache Jmeter v. 2.7^[1]
- approach 1: (1) Add config element: HTTP request defaults (2) Add Sampler: HTTP request (3) Add Listener: View results tree (4) Run the test plan
- approach 2: (1) install unofficial jmeter-plugins (2) Add Sampler: jp@gc - HTTP Raw Request^[2] (3) Add Listener: View results tree (4) Run the test plan
Packet sniffer
Poster [1] for Firefox
Fiddler Web Debugger - A free web debugging tool for Win

Firebug for Firefox : Menu -> Net
LiveHTTPHeaders for Firefox
Masquerading Your Browser: also offer the tool HTTP Header Viewer [Last visited: 2012-05-16]

Web page compression check

online gzip test