The tools that I could not do without are:
LiveHTTPHeaders is an extension that lets you see what headers are being exchanged in web transactions, like when you load up a site, you can see what is being requested. Now that may not sound too impressive, but this extension lets you see what’s really being sent, allowing you to see why you can’t send a query string directly to a particular address. Particularly useful for searches, this extension allows you to see POSTDATA, which can then be sent from your extension. Cool.