Internetworkers: http://www.ibiblio.org/internetworkers/

[ron thigpen <ron AT fuzzsonic.com>]

If all you need is a more fully featured JS debugger to work with your 
favorite IDE, then you may want to try Venkman.  If you've not used a 
real debugger before then there's a bit of a learning curve.

http://www.mozilla.org/projects/venkman/
http://www.svendtofte.com/code/learning_venkman/

For a JS IDE, I'd second the Eclipse recommendations, and add Aptana to 
the list.  It's Eclipse based, but a bit more approachable in that it 
doesn't force a project-centric world view.

http://www.aptana.com

--rt