Longtimers with Mac OS X remember that transparency came early with the first version of the Aqua interface, but soon it went away, Now, transparency is all over the UI in Mac OS X 10.5 Leopard.

A transparent Mac SE, made for internal use only, fails to reach its minimum $25,000 bid price on eBay and the auction ends. Rare, transparent Mac SE not worth $25,000 - CNET X ...