Firebug
TamperData
Venkman
http://developer.mozilla.org/en/docs/JavaScript_Shells
http://kodfabrik.com/app/pi.debugger/
Microsoft Script Debugger
Command line shell for JavaScript
MSIE Developer Toolbar
Debug Bar
DOM Level 3 XPath Library. This is not a debugger, but may help locate elements, examine DOM. IE 7 and previous version does not natively support XPath. Javascript-XPath home
Memory Leak Detection
To detect memory leaks in IE, you can use a program such as Drip which automates the testing process, or test manually using Task Manager or Process Explorer which allows detailed monitoring of a single process.
As mentioned on the Drip homepage, a leak is detected by looking for a rising stairs pattern as a result of reloading the page. For example, if I load http://www.yahoo.com and notice that IE is taking 40MB of RAM, reloading the page 10 times should still show IE as taking 40MB of RAM. However, if you notice that after each reload, and additional 2MB is taken up resulting in a pattern such as 40MB, 42MB, 44MB .. 60MB after 10 reloads, then the page in question is leaking.
