Mac OS X: It's rare, nowadays, to find any theme for Mac OS X 10.6, let alone a great one, but thanks to the hard work of deviantART user mrtomcraig, the long standing-ly great theme Amora has made its way to Snow Leopard.
Mac OS X's interface has long been a work in progress. Aqua started out as the dominent theme, and slowly each major update has eroded its various user interface elements. While Mac OS X 10.7 Lion will mark the death of many of Aqua's remaining elements, Snow Leopard still contains several remainders (like the scrollbars, for instance). Amora does a wonderful job of unifying system elements and providing a beautiful, unobtrusive, light-toned theme.
Amora for Mac OS X 10.6 is still in beta but is nearing completion. You can grab a copy of the beta over on mrtomcraig's deviantART page.
deviantART Thanks for the tip, Jack!