XRG is an open source system monitor for Mac OS X. XRG allows you to monitor CPU activity, memory usage, battery status, machine temperature, network activity, disk I/O, current weather, and stock market data. The goal of XRG is to be a functional system monitor that has a clean and flexible interface, so it doesn't intrude on the rest of your Mac experience.
"When things get busy...this little application is far more colorful. And far more useful: your coworkers, relatives, loved ones, and complete strangers will be impressed. It doesn't make a bit of difference what your Mac is doing: the graphs show that it is working hard and, presumably, so are you."