SLiM, or Simple Login Manager is a simple desktop login manager available for Ubuntu. Although simple, you can configure it to how you like with themes, and configuration files.
SliM comes with the following default theme:
...Already think it looks sexy? Well, I have found a page with many SLiM Themes. Defiantly worth checking it out! And if you can't find a theme that suits your needs, have a go at making one. I have yet to make a theme of my own, but as soon as I have some spare time, I will create on and see what it turns out like. I'll post it on here for everyone to share :)