The Yahoo Widget Engine (formerly known as Konfabulator) lets you run all sorts of fun and useful mini-apps known as Widgets anywhere on your desktop. There are thousands of widgets already available from Yahoo!, and if you don't like those you can also build your own.
Some of the more useful Widgets that are available are: clocks, calculators, calendars, day planners, etc. There are also several useless widgets that are really fun too. Check out the gallery to see what is available.