Geocons is a nice collection that includes 75 simple icons inspired by the Geosans font. Geocons collection is great if you want to give a clean organized look to your Mac.
Every volume (disk), application, file, and folder on your Mac is represented by an icon. Volume icons tend to look like the item that they represent, for example, your hard disk looks like a hard drive, CDs and DVDs look like discs, and removable drives look like removable drives.
Geocons icon set allows you to give an original touch to your desktop no matter the file types or applications you choose to associate.
To learn mode about changing your default Mac OS X icons you can access the Apple Support page HERE.