adj. Describes a software program that is capable of using different modules — called skins — that change the program's colors and interface design.
Winamp is skinnable so you can change the look of the software as you wish. The Winamp site purports to offer 12,500 skins so you should be able to find something you like.
The other noticeable feature is the interface. If you find a skinnable player — one whose interface you can change at will by downloading a new look — you're not tied to the standard look and feel.
Literally thousands of skins are available at sites like Boyda's Skinz.Org and Customize.org. And there a host of "skinnable" apps — like NeoPlanet and WinAmp, which have made their interface source code available to skin designers.