The great folks over at the Yahoo have a TERRIFIC set of stencils for UI design. They cover the following topics:
* Ad Units
* Calendars
* Carousels
* Charts and Tables
* UI Controls
* Form Elements
* Grids
* Menus and Buttons
* Mobile – General
* Mobile – iPhone
* Navigation and Pagination
* OS Elements
* Placeholder Text
* Screen Resolutions
* Tabs
* Windows and Containers
They are available in Omnigraffle, Visio and PNG format, so
go get them at the yahoo developer site