Mac OS X only: Not keen on Leopard's default theme, or just prefer to ditch the status quo for something more your style? Free application Mangifique quickly and easily swaps custom themes in OS X.
Magnifique is surprisingly easy to use. After you install it, you can browse through a handful of available theme downloads to find one that suits your fancy. Once you've chosen a theme you'd like to try on, just download it via the Magnifique browser and apply it. You'll want to restart Finder, your Dock (don't worry, the app comes with buttons to do this for you), and any other application that you want to see the newly applied theme with—and voila, you're rolling with a custom theme.
As with any serious, system-wide customization tool, you should approach Magnifique with some amount of caution. We haven't thoroughly tested it yet, but we were able to successfully apply a skin and restore to our default without any problems, so Magnifique is passing the test so far. If you give it a go, let's hear how it's working for you—and what themes you're liking—in the comments. Thanks Tobiah!