Y!Slow
Some background -
http://ajax.phpmagazine.net/2007/07/yslow_helps_you_speed_up_your.html
http://developer.yahoo.net/blog/archives/2007/07/yslow_release_o.html
http://www.phpied.com/yslow-performance-extension-for-firebug/
http://corfield.org/blog/index.cfm/do/blog.entry/entry/Improving_site_performance_with_YSlow
Firebug -
Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
YSlow -
http://developer.yahoo.com/yslow/
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