How Many Visitors Are Viewing My Site?

Is there a method in ASP, which allows you to note how many users are currently viewing pages on your site.

I understand that there is a way of knowing if the user has actually logged onto the system, as I am sure you can detect how many members are actually signed in using I think global.asa file If I am right. I am sure you could also do it via an sql query if memberid=true, I am sure you could output how many members are true, returning a numbered value.

But is there a method of detecting how many viewers are actually connected to your site, without them actually being logged in? i.e non-members.

Also, how many hits to the site would be good, however this is an inaccurate reference, as a simple refresh would add 1 to the value, is there a method to prevent this, off the top of my head I would think a detect via the ip address? however some ip addresses change depending on there ISP.