Just a word of caution: this app is only for people familiar with UNIX—a fact that the developer is quick to point out in the Mac App Store listing—but if you’re not intimidated by shell commands, Tysoe.org’s GeekTool (Free) can pull off some pretty cool tricks. It can display information right on your desktop, eliminating your need to go to a separate app. Three different default plug-ins are supported.
The first is a file plugin to monitor Mac OS activity, the second is a shell mode for launching custom scripts or commands, and there’s also an image mode that helps you monitor things such as memory availability and CPU load. If you don’t even know what those mean, this probably isn’t the right app for you. But if you get the concepts and are willing to learn or Google for answers, then you could definitely have some fun: as the screenshot shows, you can create a huge clock on your desktop, have a weather forecast running without a Dashboard widget, and put other little nuggets of system information at easy reach. These are the kind of customizations that make people stop and say “How’d you do that?”