You may have seen the lovely mini icons set Diagona (400 icons are included in PNG format) from Pinvoke. Now Pinvoke has released another great set of mini icons - Fugue Icons. There are 422 icons are included in PNG format. The icons can be used free for any personal or commercial projects. However, if you would like to remove the attribution, you should purchase the license.

Requirements: -
Demo: http://www.pinvoke.com/
License: Creative Commons 3.0 License