.. might be Google Analytics. Google bought out Urchin last year and they have released the popular website statistics package for free. Until recently registration was limited to an invitation system but now anybody can sign up.
Most website hosting comes with a preinstalled statistics program and you really should go through the stats occasionally to see what people are actually looking at on your site - if the popular pages aren’t linked from your homepage I would suggest rethinking the navigation structure you are using. The advantage to GA is that it gives you so much good information where other programs can be lacking.
There have been some criticisms about the reliability of the data but for most people the service should be fine. It is free, if you need super accurate results you can always still buy Urchin.
If you are one of those people who is learning more about search engine optimisation there are a few tips at the Official Google Analytics Blog.
Update time! After acquiring MeasureMap it was only a matter of time until the awesome interface work appeared in a Google product. Just recently Google Analytics got a facelift and the changes have made a huge difference to how useful the program is. If you have a website and you haven't tried it yet, go over and sign up.