PsLoggedOn

PsLoggedOn is an applet that displays both the locally logged on users and users logged on via resources for either the local computer, or a remote one. If you specify a user name instead of a computer, PsLoggedOn searches the computers in the network neighborhood and tells you if the user is currently logged on.

Source : http://technet.microsoft.com/en-us/sysinternals/bb897545.aspx