Internet Explorer Developer Toolbar
http://www.microsoft.com/downloads/details.aspx?FamilyID=e59c3964-672d-4511-bb3e-2d5e1db91038&displaylang=en
For FireFox
Venkman Javascript Debugger
http://www.mozilla.org/projects/venkman/
WebDeveloper Extension
http://chrispederick.com/work/webdeveloper