The amazing Noun Project collects universally recognizable symbols and makes them free to the public. Browse through featured symbols collections or browse icons by category, and when you find an icon you like, you can download it for free as an Adobe Illustrator file. The Noun Project pledges to keep these resources free forever:
We think a language that can be understood by all cultures and people is a pretty amazing thing. We also think our symbols and the objects or ideas they represent are works of art worth celebrating. … We get excited about things like scale, proportion, and shape. We are committed to design and quality in everything we do.
The gorgeous site was built by Simple Honest Work out of Chicago.