Winamp
Predating the iTunes madness that runs rampant today, Winamp is a music playing application for CDs and other digital forms of music on your computer. The best part about this program is the fact that it is completely customizable. The "skin" is the outside appearance of the application that can be changed in the same way you change the colors and wallpaper of your desktop. It comes in .zip format, and when the skin is unzipped, it is full of smaller pieces of images that fit together to make the entire application. (View a how-to video here on YouTube!)
Due to the massive amount of skins I've designed, and the fact that Winamp has tragically fallen out of popularity, I will simply show thumbnails of every skin I've made with links on where to download them, where applicable.
bravenet.com