Iconic creates beautiful custom folder icons
that fit in perfectly with the standard OS X folder icons.
Drag and drop a black and white image into Iconic and it will turn it into a folder icon for you. To add your new icon to a folder just drag and drop the folder into Iconic. It couldn't be easier!
Please note that Iconic attempts to create a folder icon that is in keeping with the built in icons, so using simple black shapes on a white or transparent background will give you the best results.