Yahoo! Widget Engine for Mac OS X 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 AirPort signal strength, will fetch the latest stock quotes for your preferred symbols, and even give your current local weather.
Version 4.5.2: Release notes were unavailable when this listing was updated.
Mac OS X 10.4 or later Leopard compatible.