I recommend a software pick of the week in each episode of the Windows Weekly podcast, but this one is particularly nice, so I wanted to toss it out in case you're not listening. It's called Windows 7 Logon Background Changer, and as the name suggests, it lets you easily replace the Windows 7 logon screen background with an image of your choosing.
Windows 7 Logon Background Changer is a free open source software that let you change the wallpaper of the Windows 7 login screen. It has been tested successfully with the final version of Windows 7 (RTM) and Windows 7 RC 7100.
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. It runs fine with my Radeon HD 3200 on my laptop computer, but it should also work with lower end graphic cards, albeit slowly.
Developers can also download the source code. Thanks to Michael for the tip!