I would suggest Rainmeter.
To quote the little blurb on their site:
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.
Rainmter makes it easy to keep an eye on information such as:
- battery power
- online data streams
- RSS feeds
- weather forecasts.
Rainmeter is free of charge and licensed under GNU GPL v2
For a good place to start on designing your own apps, I would suggest reading the Rainmeter manual
Otherwise I would recommend visiting the Discover page to find a theme from one of the available sites such as customize.org or Deviant Art.
A few example themes (click for source/download):
compilation source and more themes: http://windows7themes.net/8-free-windows-7-rainmeter-themes.html