<p>Is there a way to determine the number of users that have active sessions in an ASP.NET application?  I have an admin/tools page in a particular application, and I would like to display info regarding all open sessions, such as the number of sessions, and perhaps the requesting machines' addresses, or other credential information for each user.</p>

The way I've seen this done in the past is adding extra code to the Session_OnStart event in the Global.asax file to store the information in a session agnostic way, e.g. a database or the HttpApplicationState object. Depending upon your needs you could also use Session_OnEnd to remove this information.
You may want to initialise and clean up some of this information using the Application_Start and Application_End events.
The administration page can then read this information and display statistics etc.
This is explained in more depth at
http://msdn.microsoft.com/en-us/library/ms178594.aspx
and
http://msdn.microsoft.com/en-us/library/ms178581.aspx
.