I have never seen a hack or addon released with this..
but I was actually going to do this for my site..
where it shows how long a user has been on.. for that amount of time before they log off.. and then it would keep track of a total amount of time they have spent on my site..
kind of curious about a few members of my board... seems they either are really bored and sit on it all day.. or what.. dunno.. will find out