WinVistaClub releases OneWorld theme for Windows 7
Apart from the official Microsoft options available on the Personalization page, WinVistaClub and IntoWindows have put together the OneWorld theme.
OneWorld features five wallpaper images (compiled from other sources) and makes use of Windows 7's desktop background slideshow feature. It's designed to present an ever-expanding view of our place in the universe, from a cityscape, to a view of Earth from space, to the more abstract finale. The theme also tweaks the Welcome Center and Explorer's task pane.
Installing the themepack does require that you replace two dll files in your System32 folder. If you're not comfortable doing that, you can still install the wallpapers by extracting them from the themepack with a program like 7zip.
Note for 64-bit users: the patched dlls provided in the zip archive are from a 32-bit install, and likely won't work on Windows 7 x64.