Windows
7 includes five themes that will help you customize Windows to fit your
taste. If you are tired of the default themes there are a few sites
where you can download more. Check out these sites:

• The Official Microsoft Windows 7 Personalization site
  
• iThinkDiff.com's Windows 7 Themes
  
• IntoWindows Windows 7 Themes
  
• Mike Swanson's Themes
  
  

I will update this list as more Windows 7 theme sites are discovered. If you know of one, please let me know.