iPulse is an application that graphically displays the inner workings of Mac OS X. Its user interface displays a multitude of information on the desktop or in the Dock. The entire UI is completely configurable so you can turn off gauges you don't want, leaving only what you're interested in for easy viewing. iPulse gauges CPU activity (single and dual), system load, network activity, memory activity and usage, disk usage, and the current time and date.
Version 2.1.9 adds support for reading temperature from SMC on the latest Intel Macs.