ShapeShifter
ShapeShifter is an Application Enhancer (APE) plugin for Mac OS X developed by Unsanity that allows the user to make system-wide modifications to the appearance of the operating system's graphical interface by applying GUI skins through "injection" into running code and without modifying system files, very much like Kaleidoscope or WindowBlinds. It uses GUIKit files (called "themes") which can contain interface elements, desktop backgrounds and icons. GUIKit files are most often edited using a program called ThemePark. Like many APE modules, it does not work with 10.5 or 10.6, due to major changes in the way they  draw interface elements as compared with earlier versions of OS X. Development on ShapeShifter has ceased, with the rationale from ShapeShifter's primary developer being that he feels the fashions in which 10.5 and 10.6 draw graphics are stopgap measures in place of a new Core UI API, and so is waiting to rewrite ShapeShifter until OS X's GUI subsystem has restabilized.