The Noun Project is a great collection of clipart of universal icons. I looked at the collection eighteen months ago. This morning a post on Cool Infographics got me to look at the Noun Project again. The catalog now contains twelve categories of public domain, Creative Commons, and royalty-free icons. Learn more about the Noun Project in the short video below.

Building a Global Visual Language from The Noun Project on Vimeo.

Applications for Education
If your students need simple images for illustrations, diagrams, or multimedia projects, the Noun Project is a good resource for them to browse through.