GeekTool is a System Preferences pane for Mac OS X that allows the user to make an arbitrary number of customizable windows overlaying the Desktop, showing logs and other text files, results of CLI commands, or images (e.g. from webcams or server reports). Rather like frameless term windows.
Upon the release of Tiger, however, GeekTool suffered from a framework-related bug and ceased being usable, unfortunately for the many geeks who’d gotten used to depending on it. The developer refused to release a 10.4 update, citing the need to work on paying projects, and so the program has fallen by the wayside for Tiger users for some time now, replaced by other, less elegant solutions (like Dashboard or Konfabulator Widgets).
To the rescue came JAW software, updating the open-source GeekTool in an unofficial branch that removes the offending function that was causing the problem, as well as fixing other bugs. The JAW build is available from their OS X miscellanea page.
Some of the conversation between Tynsoe and JAW can be found in this MacOSXHints story, Install a modified Tiger-compatible version of GeekTool. See also this recent 43folders post by Merlin Mann, GeekTool’s new Tiger compatibility (and using it to build your own Batcave) for some cool uses.
Back in the pre-Tiger days, I designed a couple of GeekTool monitors: Mini-Monitor, which displays unread counts for Mail and NetNewsWire in a tiny window, and iTunes Monitor, which combines GeekTool and the iTunes Command Line Control utility to get plaintext control over your musical library.