With The Noun Project, the United Nations has made their library of symbols public.
From the site:
Symbols are some of the best communication tools we have to overcome many language and cultural barriers. When a disaster strikes, it is vital that the humanitarian community can gather reliable data on the locations and needs of affected people and who is best placed to assist them. This often involves the need to present complex information in a way that everyone can understand. By making these symbols easily accessible on The Noun Project, the United Nations is helping humanitarians, disaster responders, and everyone around the world quickly and easily communicate some of the most crucial and complex concepts, no matter where they are.
The library contains thousands of symbols in SVG format. There are several featured collections. Some, such as the AIGA and National Parks Service collections will be familiar, while others are a bit more obscure. If you are looking for some iconography for a project, The Noun Project is a good place to start.
“Slip Hazard” symbol by Joel Burke, from The Noun Project collection.