Ext JS 3.0 comes with extensive documentation. The documentation viewer is itself implemented with Ext JS, providing an excellent ExtJS example.
Ext JS lets you create GUIs that look just like many desktop applications, complete with tabs, trees, resizable panes and so on.
The latest version of Ext JS supports several DataView type controls, like the one here. What you can't see in this static picture is that the items in the DataView are selectable and can even be dragged around.
As seen with this grouping grid, grids are as complete as they are for any desktop development system.
This is an example of a portal-style application. The boxes on the right can be dragged around and rearranged.
One cool feature is the combo box can use AJAX to fill in the drop-down list on the fly. Rendering is performed using a template that the developer specifies.
You can even create a desktop-style application right inside the Web browser, with icons; draggable, resizable windows; and a Windows-like start menu.