Mac OS X Interface Stencil Kit
Leopardy interface stencil kit. This one makes extensive use of tables, for maximum flexibility while maintaining pixel precision.
Facebook Applications Stencil Kit
A pretty sizable collection of Facebook related elements to use in creating wireframes for Facebook applications.
Flex Stencil Kit
Includes all Flex components from Flex 3 Style Guide: panels, data grid, buttons, fields, links, toggle, menu, scrolls, accordion, tabs, list, data picker, tool tip, errors.