Each month Smashing Magazine, puts together a set of desktop wallpaper for the month with calendars on it. These images are from various worldwide contributors.
TimHeur.com has put together a Windows 7 themepack using these wallpapers.
You might also want toc check out the wallpapers at SmashingMagazine.com.
Create your own themepack using these cool wallpapers!
UPDATE: It is now covered on The Windows Blog too.