Windows 7 Logon Background Changer is free open source software that let you change the wallpaper of the Windows 7 login screen (also known as "welcome screen", "login screen" or LogonUI).
On a side note, this small program is WPF based, it's a nice technical demo of Windows Presentation Foundation capabilities for those interested in WPF. It requires a decent GPU for the 3D animations to run smoothly.
