Pretty damned essential system monitor. Keeps track of up to eight different aspects of system usage right from your menubar:
-CPU and Memory usage
-Date & Time
-Fans & Power
While you may not be interested in the exact goings-on with some of these (bluetooth and drives namely), iStat Pro is fully customizable. Just which aspects of the system are shown and hidden at your choosing and you can even change the fonts and graph colors - all from a handy prefpane (located in the "other" section at the bottom of System Preferences).
I prefer the menus version, though the program is alternatively available in 2 widget forms (I'm a pretty light dashboard user myself).
It's kept me from melting my CPU, from overpowering my virtual memory capacity, even helped me diagnose a connectivity problem that Airport troubleshooting was no help on. Also, the pull-down calendar has a real leg up on the Leopard date and time.