Centreon is an open-source, fully evolved network monitoring tool. Available in ISO and OVA formats, Centreon is a compatible alternative to historically powerful PRTG and MRTG tools.
I'm curious how many domain owners use Google Analytics to see who's coming to their site. It's free to get started, simple setup that Google walks you through adding a custom DNS record they use to verify your ownership of the domain. Once in, the results are gorgeous. The metrics provided are pretty powerful also. They can be used to fix sections of the website that are not being seen. It also shows the behavior of user activity on your site. Another great Google App.