Rainmeter would be without a doubt the best free Windows desktop customization utility, just have a look at the Windows 8 metro UI themes shared earlier and you will know it's not just a software utility but more like a platform having powerful capabilities.

Rainmeter Skin Gallery is a free, portable software which allows you to browse, preview and install 240+ "Free Rainmeter Skins" without leaving the Rainmeter Skin Gallery interface, the utility also shows the skin developer’s name, link to developer's webpage, supported rainmeter versions, skin release date, download size, and rating for an informed downloading decision.

Rainmeter Skin Gallery is not only one of it's kind utility, but also the only one offering the most easiest, fastest and widest choice of Rainmeter skins at a single location.