Here's one way to do this:
http://madskristensen.net/post/Track-your-visitors-using-an-HttpModule.aspx[
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At a very elementary level you could write a parser for your HTTP server logs.
A slightly more involved approach would be to do this in the server side code. There may be existing libraries that help with this. But eventually it comes down to saving the IP address and/or logged-in user profile of the visitor (if you need that info) along with a date-time stamp.