X display manager

 xdm manages a collection of X servers, which may be on the local host or
 remote machines. It provides services similar to those provided by init,
 getty, and login on character-based terminals: prompting for login name and
 password, authenticating the user, and running a session. xdm supports XDMCP
 (X Display Manager Control Protocol) and can also be used to run a chooser
 process which presents the user with a menu of possible hosts that offer
 XDMCP display management.