LightDM

LightDM is a cross-desktop display manager whose aim is to be the standard display manager for the X.org X server. The key features (as listed by upstream) are:
 * A well-defined greeter API allowing multiple GUIs
 * Support for all display manager use cases, with plugins where appropriate
 * Low code complexity
 * Fast performance

Stable version
Install :

Testing version
Starting from version 1.1.3, LightDM does not build any greeter, so you need to install them separately. The following greeters are available:
 * - A GTK+ based greeter for GNOME or Xfce
 * - A KDE greeter
 * - A Razor-qt greeter

Installing a greeter, e.g the GTK+ greeter, will also pull in LightDM itself. The USE flags of are:

Configuration
The configuration file for LightDM itself is:

KDE
The LightDM KDE greeter also provides a KCM (KDE Control Module), which appears in the System Settings under System Administration → Login Screen (LightDM)

Due to LightDM currently assuming GTK+ greeter as the default, in case you want to use the KDE greeter, you need to explicitly specify it in the :

RazorQt
The RazorQt greeter currently does not offer any configuration options.

Boot service
To have LightDM start at boot change your :

Finally, add xdm to default runlevel: