This clip art collection is in the public domain, which means you can use it for projects without worrying about infringing copyright laws. Lots of wacky, colorful images in a variety of subject areas.
Fantastic place to begin looking for public domain images. Use the search box and then click on Images. All results should be public domain since they are from the government. Another way to search for government images is to do a Google search on your topic and then type in site:gov, which should give you government sites. Be sure to check licensing though.