Windows only: We've seen utilities to change up the background picture on Windows 7's logon screen and insert Picasa and Flickr pics, but one deviantArt user provides a file swap that changes the logon box itself back to an early-stage "Longhorn" appearance.
To swap out Windows 7's small, square logon box for the more rectangular and glass-framed look it had back in the early days of Microsoft's Longhorn drafts for Vista, CyberNet News explains how to make the swap, using a DLL file picked up from a crafty deviantArt tweaker:
The file you will be replacing is the authui.dll that is located in the C:WindowsSystem32 folder. However, before you'll be able to replace the DLL you'll need to take ownership of the authui.dll file. To do so I highly recommend using the registry script here, which will add a "Take Ownership" option to the context menu. The download includes a second script that you can run to remove the option from the context menu after everything is done, or I'd say it is useful enough to keep around.
Neat stuff, explained in full at both the source page and at CyberNet. Found another handy logon switcher? Tell us about it in the comments.