LightDM

LightDM is a free and open-source X display manager that aims to be lightweight, fast, extensible and multi-desktop. It can use various front-ends to draw the user interface, also called Greeters. It also supports Wayland.
LightDM is the default display manager for Edubuntu, Xubuntu and Mythbuntu since the 11.10 release, for Lubuntu since the 12.04 release until 16.10, for Kubuntu beginning with 12.10 until 15.04 for Linux Mint and Antergos.

WWW Official Site

Features

Features include:

Codebase with very few dependencies
Supports different display technologies
Supports remote login
Comprehensive test suite
Standards-compliance
Well-defined interface between the server and user interface
Cross-desktop
Well-defined greeter API allowing multiple GUIs
Support for all display manager use cases, with plug-ins where appropriate

LightDM has a simpler code base than GDM and does not load any GNOME libraries to work, but at the cost of some features that the user may or may not need.