Action Pants

I love Firebug but it only works for IE. They do have Firebug Lite for other browsers but it doesn't always work with all applications. In my searching I found a tool called Companion.JS. I use it sometimes when debugging issues with my CRM JS (You have to access CRM using the http://[server]/loader.aspx page to do this). For the most part it helps me get the job done. I find that depending on the situation it's quicker then using the debugger statement in my code.
Keep in mind that this doesn't handle debugging JS that has been loaded into the document dynamically so if you're including external JS in your onload event then you need to place it directly in the onload event while debugging. Hopefully they add more features in the feature.