How do I access all active sessions on the server?

You can't access one user's session from another session (even if you are physically at the machine). However, you can follow our cart example to some extent, and store all the session data in a database. Keep a column called 'active' which indicates whether the user is still there. When the user logs out (or after a specified time period, which you can schedule using a job in SQL Server), you turn the active flag to off. Passing a single integer session variable around, and querying the database when you need to, is much more efficient and scalable than storing all those values in session variables anyway (even if you need all the values on every single page!). I just finished re-working a very major application to handle things this way (partly for performance, and partly because the session variables were becoming unruly and unpredictable). 
 
Remember, don't use sessionID as a key, and expect to uniquely track users (see Article #2085 for more info). 
 
If you're interested simply in counting the number of active sessions on the server, see Article #2491.