Google Analytics

WOW, they are very impressive. Easy to set-up (if you've already got a Google Mail account, just add it to your services). Then Google give you some code and you paste it into your web page, Google does all the rest.

It's all graphically represented, nice pie charts/graphs/etc. The overlay piece is very cool. It shows where people are coming from, how they got there, what they're looking at and more.

If you want to be anal about your site statistics, Google Analytics is the way to go. Really WOW.