Ubuntu 9.04 actually comes with a decent looking Login Window. However, if you like to change things and customize them, then you should go to Gnomelook to get some cool eyecandy. To download a new login window for Ubuntu click on GDM Themes on gnomelook. You can browse the various themes by most recent or just click on most popular. There you will find some very aesthetic looking themes. Download the login window you like the most. Then click on System, Administration and Login Window. Click on the tab that says Local. There you can add your file that you downloaded. Once you added the new theme make sure to select it. Just reboot your Ubuntu Linux and check out your cool new login screen.