trying to figure out the best/efficient way to tell who is logged into a site..

hi... anybody have pointers to trying to tell who/how long someone is logged into a system/site. i've thought about setting a session var, but i'm not sure how to read/tabulate this var across the entire group of people who'd be logged in. i've also thought about keeping track in a db tbl.. however, i'm still not sure that i've got a good way of tracking who's logged in, and still on... a possible approach would be to have the app periodically update the system whenever a logged in user goes from page to page... so, any thoughts/ideas/etc... -thanks bruce bedouglas@earthlink.net