Dashboard

Dashboard is a unique application for Mac OS X, released with the 10.4 version of the OS, codenamed Tiger. When opened, it superimposes a translucent layer over the desktop, and displays a number of mini-applications, some of which can be downloaded from the Mac App Store (a new feature coming in the soon-to-be-released Lion version of the OS, currently in testing.), or from the Apple website. These mini-applications are usually specified for specific tasks, like unit conversion, weather forecast, playing simple games (a lot of which run on the Unity engine) and more.

Trivia
If you use the application Terminal, and enter the following code, you can activate a special mode in Dashboard.

defaults write com.apple.dashboard devmode YES

then, after executing that,

killall -HUP Dock

Now, go into Dashboard. Select any widget, and hold. Then press the key that exits Dashboard. If you did it correctly, the widget should stay in your cursor, not disappear like the others. You may now place it as if it was a Windows 7 widget.