Glass Onion is a beautiful Windows 7 theme by deviantART user SolMiler, that's really two themes in one: dusk and dawn. It features a similar glassy look to the standard Windows 7 user interface, but smoother, simpler and cleaner.
Glass Onion also comes with unified, distraction-free icons for your task bar and an attractive clock widget. You have choices between two system fonts and it supports multiple task bar locations. Because this theme applies a lot of customisations to your system, you'll need to do a little bit of work to get it installed. For the full instructions, and to download the theme, visit the deviantART page.
Glass Onion [deviantArt]
Thanks for the tip, Chris!