Yahoo! Design Patterns Library has a set of “stencils” available to mock up your next great RIA project! The stencils are available in OmniGraffle, Visio, PDF, PNG and SVG formats. So you can use them with Illustrator or Photoshop / Gimp as well.
They have included stencils for: 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.
That’s quite a lot – all available under Creative Commons.
And if you were building Flex RIAs, here’s an article on using these stencils along with Flash Catalyst to churn out kickass Flex Apps with almost zero coding!
Enjoy!