YSlow: Speed up your web pages

YSlow analyzes web pages and tells you why they're slow based on the rules for high performance web sites. YSlow is a Firefox add-on integrated with the popular Firebug web development tool. YSlow gives you:
Performance report card
HTTP/HTML summary
List of components in the page
Tools including JSLint

Learn more and download YSlow here: http://developer.yahoo.com/yslow/