Rainmeter is the best known and most popular desktop customization program for Windows. Enhance your Windows computer at home or work with skins; handy, compact applets that float freely on your desktop. Rainmeter skins provide you with useful information at a glance. It's easy to keep an eye on your system resources, like memory and battery power, or your online data streams, including email, RSS feeds, and weather forecasts.
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.
Rainmeter is open source software for Windows XP/Vista/7 distributed free of charge under the terms of the GNU GPL v2 license.
Tip - Don't forget to check the Rainmeter Skin Gallery.