Awhile back when Ext 3.0 was still being built, I saw a video of an Ext JS designer, which looked similar to Flex Builder. I’ve been waiting its arival ever since.
Today they released a preview version that allows you to experiment with the designer interface and to explore how configs affect your layout. They say soon you will be able to build your application components using base Ext components and Certified user Extensions. The application is an Air App making it platform independent if you have Adobe Air installed for Windows, Linux, or Mac.