Ulog enables you to list past and present X11 sessions on a whole domain of machines. It is the X11 equivalent of "who" or "last" commands, with much more sophisticated options and search criteria (such as login username, hostname, display name, login/logout times, etc.). It relies on a ulogd daemon which keeps track of events (login, logout, etc.) sent via, for example, the startup and reset scripts of xdm.