Shift allows you to load any image and edit it as a replacement for icons on your Mac, including system icons and folders. While there are some performance issues and the interface is not always immediately clear, the effect is very good and the changes you can make are sweeping, giving you significant control over much of what your OSX desktop and libraries look like.
After installing Shift, you can add it to the Applications library. You can then load any image from your hard drive, which is icon and logo compatible -- this encompasses most image types though there are some exceptions you'll want to check out before using your selected file. After loading an image, you can rotate it, change its size, customize it, or edit it in a number of ways before assigning it to replace a file, app, directory, or other generic icon on your system. You can even change the boot logo icon on your Mac if you so choose. Our main issue with Shift was the performance issues that occurred in loading, between menu changes, and with occasional menu freeze-ups after saving an image. They were not persistent but they happened often enough to hinder the experience to some degree.
While the process of choosing, editing, and saving your edited icons and logos can be a bit frustrating the first couple of times you do it, Shift gives you quite a bit of control for a free app. Combined with a free downloader to choose other icons from online, it's worth putting up with the infrequent issues for the overall package.