Recently, I developed two extensions for a project I'm working on, and thought I'd share them here in hopes of getting some feedback on how they may be improved:
The Form Builder queries for a json form definition and builds the Ext.form for you. I only just today saw ServerForm so I apologize if there's some overlap.
The Cascading Dialogs extension creates various sorts of dialogs that, upon button click, check a url for a json array detailing what action to perform next (open a new window or a new panel, update a panel, create a new dialog, etc.). This is useful in providing feedback on form submission, for creating wizards, etc.
Here is an example page for your perusal, and the files may be downloaded from there as well. I apologize if the documentation is somewhat lacking. The zip includes all of the examples and I'm happy to answer any questions.