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
 * No dependencies on Gnome libraries

Installation
LightDM is available on Gentoo Linux and supports the following USE flags:

Starting from version 1.1.3, LightDM does not build any greeter, so you need to build them separately. The following greeters are available:

The following use flags are available for the GTK+ greeter:

You can install lightdm using the following command:

Install the appropriate greeter using one of the following commands:

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 as shown below:

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

Boot service
To have lightdm start at boot set the DISPLAYMANAGER variable in to lightdm, as shown below:

Finally, add xdm to default runlevel: