

Incredibly valuable for poking and prodding your pages, and seeing how your web app responds when used with slightly malicious intent. prior to posting back to a page, or getting a page. Tamper Data - lets you tamper with http headers, form values, cookies, etc. I mostly use it for quickly validating pages, disabling javascript (yes I disable javascript sometimes, don't you?), viewing cookies, etc. Web Developer - again another great developer productivity tool.

FireBug - a myriad of productivity enhancing tools, includes javascript debugger, DOM inspector, allows you to edit the CSS/HTML on the fly which is highly valuable for troubleshooing layout and display problems.
