GeekTool is a system monitor for Mac OS X, which will let you display system information or various feeds from the internet on your desktop. It’s like Conky for Linux, if that means anything to you. There are many, many scripts you can use to display weather forecasts, CPU and RAM usage, free HD space, the latest XKCD, unread emails, the track now playing, Twitter messages and pretty much anything else you can think of. As the examples I’ve attached to this post show, you can do some really creative desktops.
Thanks to Ryan for blogging about this and bringing it to my attention. His post has some great information for getting started. See also the post ‘Mega Roundup of Geektool scripts + inspiration gallery‘ on francescomugnai.com for more of exactly that.
EDIT 10.03.2010: I’ve change references of ‘preference pane’ to ‘system monitor’. The GT page describes it as a prefpane, but to me it’s a system monitor. He’s probably right, but I prefer my term.