famfamfam silk icons for prototypes

You can use contents from the Creative Commons licensed famfamfam Silk iconset for your prototypes without getting into trouble.

Mark James, a UK web developer, created the vast iconset that covers almost all icon context you may need in your project.

I personally frequent usage of the icon sets in my designs so as to better bring out the contexts of each buttons for a better user experience.

Also, Jakke has shared more iconsets with his protip