I am a big fan of Ext Designer 1.2. It saves a ton of time designing a UI. Doesn't help much with MVC or writing code though.
It has been re-branded as Sencha Designer 2 (due to Sencha touch integration), and has built-in code editing and MVC! The beta is out now.
You can try it here:
Sencha didn't link to the docs anywhere, but here they are: