The software is included in System Tools. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
Actimeter is an application that tracks your computer activity and shows it on a traditional, easy-to-read meter.
Actimeter does this by counting the frequency of all your computer interactions or events: mouse clicks, keystrokes, scrolls and mouse moves. It then calculates a score value on a scale from 0 to 100, using a carefully designed mathematical formula.
You may want to check out more Mac applications, such as Mac Ready or World Book Snow Leopard Updater, which might be similar to Actimeter.