Last Updated May 2, 2010 12:19 PM EDT
The Open Clip Art Library is an expansive collection of clipart that lives in the public domain via a Creative Commons license. All of the art at this site is available for download.
When you find an image you like - after searching by keyword - you can see what size and format the image is available in. Most assets seem to be in both PNG and SVG format; if you're using Word, PNG is the format you'll need.
In addition, you can download the whole caboodle in a single 376MB zip file. You'll get the entire library on your computer for offline browsing, but you'll lose the ability to search by keyword, like you can do from the Web site.