Custom Session Handlers and User Tracking

I am trying to develop a way of tracking users on my site. I have read a few articles on Custom Session Handlers that suggest this as a way of expanding the usefulness of sessions. I currently use sessions on every page of my site so I can track unique vistors to my site and I want to be able to display how many users are browsing the site, how many are logged in and who is logged in. Can anyone recommend and good tutorials or any further research? Or has anyone got a tried and tested method of doing this sort of thing? Rob