As you might gather from the name called GeekTool is designed for those who really want to get under the hood of their Mac.
GeekTool is a Preference Pane (so it installs into your System Preferences rather than as an external program) app that lets you display many different types of system information and processes on your desktop. You can instruct GeekTool to follow a specific file that you want to track, launch custom script commands and monitor the data load on your machine such as bandwith usage, CPU loads and memory availability.
You can instruct GeekTool to 'stay on top', so that it will always remain visible whatever you're doing. Be warned though that this is not a tool for beginners. GeekTool is pretty hard to work out for the first-time user and you're only hope is to use the GeekTool Wiki page to get up and running. There are no sample scripts or user scripts that you can test for yourself although there is a user forum if you get really stuck.
GeekTool will definitely appeal to all those advanced Mac users that want maximum control over how their Mac performs.