The PsLoggedOn utility lists either all systems a certain user is logged into, or all users logged into a certain system. See the PsLoggedOn topic in the PsTools help file for more details.
Specify either a remote host, ie
\Computer Name
... in order to list all users logged into that host, or a username, ie
Vincent
OFFICE\Gary
... in order to list all computers on the network to which that user is logged in.
If this option is selected, PsLoggedOn only lists users that are logged in locally.
If this option is selected, PsLoggedOn shows login times as well as usernames.