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Rainmeter displays customizable skins, like memory and battery power, RSS feeds and weather forecasts, right on your desktop.
Many skins are even functional: they can record your notes and to-do lists, launch your favorite applications, and control your media player - all in a clean, unobtrusive interface that you can rearrange and customize to your liking.
Rainmeter is at once an application and a toolkit. You are only limited by your imagination and creativity.
http://rainmeter.net/cms/

Customize.org is the most prominent and well-established desktop customization site on the web. Can download free themes here and addons.

http://customise.org/

Below are some examples.