iOS 5 has a nice new notification system that streamlines it all into the Notification Center. It's a nice upgrade from what the previous versions of iOS offered.
Within Notification Center are Widgets. The ones that ship pre-loaded are Weather and Stocks. Apple hasn't built in a way for developers to add their own widgets yet, but that hasn't stopped jailbreak developers from coming up with their own solutions.
Of course you will need iOS 5 and you device will have to be jailbroken in order to do it. Still, we've seen that iOS 5 is not ready for prime time (it's still in beta) so we don't suggest doing this unless you know what you're doing. Then again, you have to be a developer to use iOS 5 anyway at this point.