Gnome Display Manager
Gnome Display Manager is a re-implementation of the well known xdm program. gdm consists of three separate parts: A small daemon, a graphical login program (gdmgreeter) and a host chooser.
Light Display Manager
Light Display Manager (LightDM) is a fully themeable, cross-desktop, standars compliant display manager.
wdm combines the functions of a graphical display manager identifying and authenticating a user on a system with some of the functions of a session manager in selecting and starting a window manager. Optionally, wdm can shutdown (reboot or halt) the system.
XDM-Choose is a highly configurable replacement chooser for XDM, which allows the user to type in a hostname and query/connect to it.