Yahoo! Design Stencil Kit version 1.0 is available for OmniGraffle, Visio (XML), Adobe Illustrator (PDF and SVG), and Adobe Photoshop (PNG), and covers the following topics:

I am sure you will find them useful when designing user interface for your website / web application. All of them are licensed under a Creative Commons Attribution 2.5 License.

Requirements: -
Demo: http://developer.yahoo.com/ypatterns/wireframes/
License: Creative Commons 2.5 License