If you love downloading new wallpapers for your desktop, I suggest that you follow the @MSWindows team on Twitter because they frequently share links to new backgrounds and themes in their tweets – see examples here, here and here.
In other news, the Bing team has released a brand new theme for Windows 7 that includes a collection 21 beautiful wallpapers for your wide desktop (resolution – 1920*1200).
It’s a Windows 7 themes but you may follow this trick to install it on previous versions – How to Use Windows 7 Themes on XP and Vista.