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:

You can install lightdm using the following command:

Configuration
Configuration files of interest are:
 * /etc/lightdm/lightdm.conf
 * /etc/lightdm/lightdm-gtk-greeter.conf (when gtk useflag is enabled)
 * /etc/lightdm/lightdm-qt-greeter.conf (when qt4 useflag is enabled)

To have lightdm start at boot set the DISPLAYMANAGER in /etc/conf.d/xdm to lightdm, as examlped below:

Finally add xdm to default runlevel: