Other 2.x Extensions:

Buffering Http State Provider
DateTime Field
EditorGrid Validation Plugin
Ext.ux.clone() - Object or Array cloning function
FileTreePanel - Remote File Management Tree
Grid CellActions Plugin
Grid RowActions Plugin
Grid Search Plugin
IconCombo Extension
IconCombo Plugin
IconMenu Plugin for Window/Panel
MetaForm - A FormPanel configured by metadata received from server
Remote Field Validation Plugin
ThemeCombo - w/o page reload and keeping state
UploadPanel
XDateField with configurable submitFormat
XTimeField with configurable submitFormat
XCheckbox - A nicer checkbox always submitting configurable values

Ext 1.x Extensions:

Collapsible InfoPanel/Accordion - Extension classes with Live Demo
Ext.ux.UploadForm File Upload Widget
FileTreePanel

Tutorials:

Application Layout for Beginners
Advanced Custom Drag and Drop Series
Custom Drag and Drop Series
Extending Classes in Ext
Extending Ext 2 Class (IconCombo)
Linked Combos Tutorial for Ext 2
Localizing Ext
What is that Scope all about
Writing Ext 2 Plugins (IconCombo)

Important Forum Posts:

Application Design Based On Pre-Configured Classes
call(this, ...) vs apply(this, ...)
Ext.fly() explanation
How to add tooltips to form fields
How to solve encoding problem
How to get reply to your question quickly?
Lazy Instantiating/Rendering Explanation
Login Form Security Talk
Paging Grid Example with PHP 5.x Backend
Private Messaging, The use of
Rendering grid to an inactive tab
Simple File Upload Example
Why to load store from grid render event