“Inco is a dashboard and administration tool for Mac OS X and Mac OS X Server 10.4 and 10.5. (And others, anabolics ailment soon!) Inco runs on the machine you want to monitor, and you can access it from any web browser.” It looks especially great on the iPhone. Check out the Getting Started Guide (PDF link) to see what stats it can show you and how great they look on the iPhone.
Inco started out as a subscription product, but has since decided that a onetime charge is the way to go. For $36 you can “use Inco on as many machines as you own or operate.”
You can download it at the official Inco site.