One of the things I always liked about Mac OS 8/9 was the ability to use different interface themes. In other words, you could change the entire GUI interface away from Apple's default and give your Mac a bit of a personality upgrade (which is not to say that Apple's default look was in any way bad). With OS X the idea of themes became much more difficult and, because of the more complicated nature of the operating system, much more dangerous.

Enter ShapeShifter, from Unsanity, Inc.

For the past couple of days I've had my Panther interface "shifted" to a new look, one that is designed to resemble Apple's iPod. So far I haven't experienced any ill effects or system sluggishness. ShapeShifter apparently gets around the dangers of modifying the OS X interface by not actually changing any system files. From what I've been able to find out, the application seems to intercept system calls to the interface files of OS X and reroute them to ShapeShifter's own files. Or something. Again, I don't know quite how it works, but it does.

Right now there are only a couple of themes that come with the download, but more are in development. ShapeShifter works via a System Preferences pane, and you simply choose the theme you wan to use. Pretty cool stuff!

I've actually just switched my theme to one called "Cryogenic". Here's an idea of what it looks like: