If you have a Mac laptop beginning with the PowerBook G4, one feature you may or may not know about is the Sudden Motion Sensor. If you drop your computer or it is suddenly exposed to strong vibrations, the Sudden Motion Sensor will park the hard drive to reduce disk damage--even before your laptop hits the ground. When the sensor detects your computer has returned to a stable state, it "unparks" your hard drive and you can continue computing.
What's fun about this feature is that some savvy developers have made apps that take advantage of the motion sensor for less serious purposes. Our friends over at The Unofficial Mac Weblog (or TUAW) found an app called LiquidMac ,which lets you visualize the Sudden Motion Sensor function by filling your screen with simulated liquid. You can choose what color you want the particles to be, and change other parameters for dot size, gravity, and the amount of particles. After that, simply tilt your laptop and watch as the liquid flows in the direction you tilt. It also has a full screen option for a more dramatic effect.
I suppose it's mostly worthless besides showing off to your friends (or maybe testing your Sudden Motion Sensor? Yeah, that's it!). Either way, I think you'll agree LiquidMac is pretty cool.