I recently swapped from GDM to SLiM on my EeePC running Ubuntu Lucid Lynx 10.04 and decided to make a nice simple theme for it which you can download.

SLiM - crackware theme

SLiM is exactly that, a Simple Login Manager and it lives up to it’s name. Even being as simple as it is, you can still theme it and it has a configuration file which you can edit.