Yahoo widget engine is a great application on the mac (theres also a PC version) that can display literally limitless amounts of information from the web. It has been around for a couple of years and started out being called konfabulator and was then bought by Yahoo! and the name changed to Yahoo! Widget Engine.
Once you download and install Yahoo Widget Engine you’ll find that it comes preloaded with quite a widgets to pick from such as clocks, battery monitors for laptops, WiFi signal, iTunes controllers, maps, weather, memory monitors and quite a few others.
You can see from the screenshot kind of what they look like, how they work, and just a small sample of the widgets available. The widgets sit on your desktop and are pretty good about not sucking up too much memory and will update automatically if connecting to the web for data such as weather, stock quotes, RSS feeds, etc. Or if you interested in building your own widgets, you can check out the Widgets Workshop for a good guide on how its done.
Download: Yahoo Widget Engine