Firebug
Must have tool for s-controls
Javascript development is much less painful with the Firebug addon to Firefox.
Once you have it installed, you can put
console.log(<any var or data structure or DOM element>);
throughout your code to see exactly what is going on.
However, Firefox has been getting slower and slower for me lately, particularly in Salesforce. I learned that the problem is Firebug.
Great news is that I click Tools->Firebug->Disable Firebug, and my browser is fast again! Then I turn it back on when debugging.
Get it at http://getfirebug.com/