Magnifique: Free OS X theme customizer
First and foremost, it's free. Second, it doesn't require a full restart once you apply a theme, just a restart of the Finder and the dock. Third, there seems to be a pretty good community of theme designers and users around it, which means help with designing and using themes is readily available in the forums.
Although Magnifique seems to use its own file format for themes (you can't easily snatch themes from other Mac theming apps) there are nearly 30 themes available on the Magnifique website. Even better, you can also download them from within the app itself.
If you look around other Mac themes forums, you'll find some additional Magnifique themes. I haven't run into any problems or glitches so far, but you may want to turn off the "apply custom mods" option if you're running the latest iTunes or Safari 4, because the app-specific elements of some themes aren't as up-to-date as the system-wide elements.