Yahoo Developer Network has hosted some nice assets for wireframing and other designs. These assets are Adobe Fireworks png files which are in the symbols format allowing them to be reused across multiple pages in a document.
You can copy them in the Common Library location and refresh the panel. They will be available for your design layouts or wireframes.
A screenshot of the assets is shown below:
The complete article and the files are placed at http://developer.yahoo.com/ypatterns/about/stencils/fireworks.html
Quite an impressive set!