In the pre-OS X days, Kaleidoscope reinvented the Mac interface by changing its entire look and feel. Now ShapeShifter has picked the lock on Apple's Aqua interface. You can easily activate and preview themes through a control panel in the system preferences.
The 2.x series improved how themes were installed and downloaded, finally making it easy for the average user. It also addressed some of the minor quirks in the way ShapeShifter handles Aqua's graphical inconsistencies. The results are breathtaking, and the available themes are stunning.
Several third-party and individual sites offer even more themes. We recommend ShapeShifter, the new Kaleidoscope, for anyone looking to spruce up the OS X interface.