Give your Mac a case of the Measles

Measles, from Stick Software, provides you with little bouncy colorful spheres on your desktop. They can be configured to act as CPU monitors, changing color according to how busy your machine is, or they can use a wide variety of color schemes and fill patterns. They can also change colors according to how much free memory you have, or how much free disk space is left on your boot drive, or how long your machine has been sitting idle without user input. They have lots of different options to keep you entertained, from sounds effects to transparency. More info and download link here.