Tamper Data, a cool FireFox plugin
When I debug web sites (something I'm sure everybody does, right?), I like tools that let me look at the HTTP headers and such. Too what cookies actually are set, etc. One tool I liked was the Live HTTP Headers FireFox extension. Jeff just showed me an even better one: Tamper Data. It basically does the same thing, but there are two big differences.
First, it's formatting of the data is vastly better: there is a table of entries for each request with performance and summary data. This lets you figure out where your pages are fat: like included images or js or whatever. Then you can click on each request to find out more.
Second, you can go into "tamper mode," where you can edit the requests coming from your browser before they get to the server. This can be very handy.