iPulse is the system monitoring utility by The Iconfactory that's both informative and really great looking. Keep track of disk usage, CPU usage, system load, motherboard temperature and a lot more. Plus, you can customize its look with by selecting from a large library of skins (they call them "jackets").
Today, The Iconfactory released version 2.1.4, which has been optimized for the quad G5's. It's also a universal binary, so all you folks with developer Mactels can play along. This is a free upgrade for existing users. iPulse requires Mac OS 10.3 or better, and a single license will cost you $12.95US.