LightDM

LightDM is a cross-desktop display manager whose aim is to be the standard display manager for the 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

Installation
Install :

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, if the KDE greeter is requested, it needs to be specified in the file:

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

OpenRC
Set LightDM as the default display manager:

To start LightDM on boot, add dbus & xdm to the default runlevel:

To start LightDM now:

systemd
To start LightDM on boot:

To start LightDM now:

Command line tool
LightDM includes a command line tool, dm-tool, which can be used to switch user sessions, lock the current seat, etc. To see a list of available commands, use the --help option:

For example, to lock the current seat:

Cursor themes not working
As of March 28, 2015, the stable versions of lightdm (lightdm-1.10.3) and lightdm-gtk-greeter (lightdm-gtk-greeter-1.7.0) have a bug that affects some DEs such as XFCE with setting the mouse cursor. Applications will apply the new mouse theme, but the root window where the window manager draws on does not. To get around this, simply unmask the latest versions (lightdm-1.13.2 and lightdm-gtk-greeter-2.0.0 at the time of writing) and the cursors will be set correctly.

More information can be found in Ubuntu's bug #1024482.