We're doing a fairly thorough performance/load time analysis of westernaustralia.com at the moment and I was recently introduced to YSlow in the process. YSlow originated at Yahoo! and works as a Firebug-dependent Firefox add-on.
In addition to using a combination of packet sniffing and DOM sniffing techniques to give you an accurate idea of how long the current web page takes to load, it also reports page weight, gives you information about page load weight from a primed and unprimed browser cache, tells you all about what's got an Expires header, what's got an etag, what's being gzipped, allows you inspect page components directly or at the header level, and grades the page overall. The grading system is backed up by a thorough set of recommendation for increasing the grade.
Apparently YSlow even fixes a bug with the Firebug net pane!
Definitely enough for me to add this to my toolkit--thanks Yahoo!