Omni Group’s Omnigraffle (Mac only) is an excellent tool for web prototyping. Now with Yahoo! and Konigi FREE stencil kits it’s just irresistible!
Konigi OmniGraffle Wireframe Stencils a set of shapes for making wireframes (low-fidelity web page schematics) in OmniGraffle version 5.x (Mac OS X). It consists of most of the basic elements you’ll need to create user interface specifications.
Download the stencils

Design Pattern Library by Yahoo! Yahoo’s new Design Pattern Library, this great tool is even more useful. It includes all the widgets used on a normal web page, as well as more exotic ones like calendars, carousels and sliders, as well as common screen resolutions and os widgets.