The cool YSlow plugin for firefox is discussed in detail on the Coding Horror blog. He likes it but has some warnings about taking the numbers too seriously..
This plugin evaluates the current page using the 13 rules and provides specific guidance on how to fix any problems it finds. And best of all, the tool rates the page with a score-- a score! There's nothing we love more than boiling down pages and pages of complicated advice to a simple, numeric score. Here's my report card score for yesterday's post.
Link to YSlow: http://developer.yahoo.com/yslow/