The Yahoo! Widget Engine (formerly known as Konfabulator) is a JavaScript runtime engine for Windows and Mac OS X that lets you run little files called Widgets that can do pretty much whatever you want them to. Widgets can be alarm clocks, calculators, can tell you your WiFi signal strength, will fetch the latest stock quotes for your preferred symbols, and even give your current local weather.

The format for these Widgets is completely open and easy to learn so creating your own Widgets is an extremely easy task.

Requirements:
OS X 10.4-10.5
Universal Binary