I do a lot of work with HTTP sockets, requests, CSS and other debugging. The following tools are an essential part of my toolkit when building applications and tweaking webpages.
1) Firebug (Firefox Add-On) -
- Inspect HTML and modify style and layout in real-time
- Accurately analyze network usage and performance
- Extend Firebug and add features to make Firebug even more powerful
- Get the information you need to get it done with Firebug.
2) Web Developer Toolbar (Firefox Toolbar) - Works great for revealing form fields and messing with cookies.
3) HTTP Analyzer - View the traffic that a webpage generates so you can reproduce similar requests or hack your way into a page your not suppose to see. Also great to run when you are visiting a competitors affiliate links because it captures all redirections and page sourcecode.