Mac OS X Icon Set is a free and high quality, simple styled and elegant looking icon collection for all OS X users that have a sweet tooth for icon customization.
How to change a folder or app`s icon using PNG images: Open the PNG using Preview, select all by with Command+A and press Command C (copy). Then, go to the app, drive or folder that you want the icon to be on, open the Get Info window using Command+I, click on the top-left mini-icon and press Command+V (paste).
The Mac OS X Icon Set is licensed and distributed under the terms of the Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.