Famfamfam offers us a set of wonderful and most pupular icons on the internet named “Silk” which contains over 700 16-by-16 pixel icons in strokably-soft PNG format. And all for a low low price of $0.00. You can’t say fairer than that.

Requirements: No Requirements
Demo: http://www.famfamfam.com/lab/icons/silk/
License: Creative Commons Attribution 2.5 License