I browse with Google Chrome but use Mozilla Firefox for the development tools below as the tools either don’t exist for Chrome or lack features.
Web Developer
http://chrispederick.com/work/web-developer/
http://chrispederick.com/work/web-developer/features/
Firebug
http://getfirebug.com/
http://getfirebug.com/whatisfirebug
Yslow from Yahoo
http://developer.yahoo.com/yslow/
https://addons.mozilla.org/en-US/firefox/addon/5369/
Page Speed from Google
http://code.google.com/speed/page-speed/docs/extension.html
http://code.google.com/speed/page-speed/download.html
