If you're a developer or a tester who uses to switch between various versions of Mac OS for testing purposes, you might find handy to get GeekTool to display the system version and build number. Here's how I do it:

echo `/usr/libexec/PlistBuddy -c "Print :ProductUserVisibleVersion" /System/Library/CoreServices/SystemVersion.plist` (`/usr/libexec/PlistBuddy -c "Print :ProductBuildVersion" /System/Library/CoreServices/SystemVersion.plist`)

The result is something like this:

10.5.7 (9J61)