This website was established in April of 2000, and we began measurement of "unique visits" in January of 2003. Unique visits are individual sessions, starting when someone visits our site. It is measured by our ISP and is dynamically updated, operating as a "CGI" (a database lookup from our webpage). The site visit activity varies from time to time, and is generally more active when I am teaching a course or after I have given a talk to a large group.
It may be interesting to note that the Internet site measuring service, www.alexa.com, ranks CT.net as the 7.7-millionth most visited site, which is 0.000002% of all website visits. Not too impressive, but it puts us in the top 6.4% of all websites. Who knew?