if your using mozilla/firefox, these extensions will help you out allot!

http://www.getfirebug.com/
http://users.skynet.be/mgueury/mozilla/
http://chrispederick.com/work/webdeveloper/
http://ietab.mozdev.org/
http://www.mozilla.org/projects/inspector/
http://www.iosart.com/firefox/colorzilla/