IOGraph is a free utility for Mac/Linux/Windows that tracks your mouse movements to create surprisingly beautiful works of computer art. It works in the background by recording all the moves of your mouse in pencil-thin lines on a white digital canvas. When you stop your mouse a dot forms; the longer the mouse is static, the larger the dot will be.
The screenshot below was captured from just over four hours of work (see another two hour session), during which I took a coffee break (the large blob near the middle). I have a dual-monitor setup at work, with the dock in Mac OS X anchored to the left-hand side.
Apart from being quite fun, IOGraph can also provide interesting usability data when overlayed on a screenshot of your desktop (see this example by the application’s developer of his use of Photoshop over a whole day).