The Noun Projects is a visual library of icons representing everyday things we stumble upon daily, but this library is free to the public for use under Creative Commons. Pretty sweet project, and they also have Kickstarter page if ya want to help them out.
Heres a bit more about them:
“sharing, celebrating and enhancing the world’s visual language”
The Noun Project collects, organizes and adds to the highly recognizable symbols that form the world’s visual language, so we may share them in a fun and meaningful way. Here is our pledge to you:
The symbols on this site are and always will remain free. We believe symbols can not be effectively shared with the world if they are not free.
Everyone like simplicity. We want you to be able to come to our site and effortlessly find and obtain what you are looking for. Simple as that.
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. Check out our store.
We get excited about things like scale, proportion, and shape. We are committed to design and quality in everything we do.