Here's a bit of the ol' ThemePark 4, now with Snow Leopard compatibility. If you're tired of Aqua, you can use ThemePark 4 to change your UI to whatever you'd like. ThemePark will even apply your changes safely to your operating system for you.
ThemePark 4 can:
Oh, and ThemePark 4 is also completely free, at least for now!
In the unlikely event of a ThemePark emergency, you can revert to Aqua via the command line by entering the following command:
/Library/PrivilegedHelperTools/com.geekspiff.themepark --revert
Unfortunately, I won't be updating ShapeShifter for Snow Leopard, nothing has changed since Leopard was released and so my previous rationale still applies. Fortunately, ThemePark 4, which is free, still allows some level of theme creation.
However, Unsanity is still hard at work on Snow Leopard compatibility for the rest of our stuff and will be announcing availability as things become ready. Look for posts from Slava and Rosyna about WindowShade, FruitMenu, FontCard, Labels X, Menu Master, and Xounds in the near future, and watch for something from me about Mighty Mouse.
In the meantime, happy theming!