Paul, take a look at Level 10's counter.
http://www.leveltendesign.com/L10Apps/HC/
It is the most advanced and powerful counter I've found. It goes pretty damned far at tracking your visitors activities. It's free, but it is also not invisible. I'm sure they can modify it for you for a few smackers if their tiny logo at the bottom of your page bothers you. You can click on the WhoringWives link in my sig to see what it looks like at the bottom of my pages next to the sextracker. It's not a third party counter like sextracker though. You install it on your site and it uses a mySQL database. If you use php included footers you only have to paste in the tracking code once and it will track every individual page the footer is on. Just easier that way.