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.
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.