A tool that network administrators use?

Does anyone know of a tool that network administrators use to determine which workstations users are logged onto in an NT environment? Something ?simple? with a nice interface that lists all of the machines in the domain and what users are logged onto them at that moment in time. If it had more advanced features that had logging to generate simple historical reports that list username, and resources used either locally and/or domain wide would be nice but not essential. Oh yea, something that?scheep or free is even better. Thanks in advance.