StatCounter

If your web host doesn't offer a stat counter, this one was highly recommended to me. "A FREE yet reliable invisible web tracker, highly configurable hit counter and real-time detailed web stats. Insert a simple piece of code on your web page or blog and you will be able to analyse and monitor all the visitors to your website in real-time! Free, Fast, Responsive, Quick loading and Reliable Service. Invisible Tracking - no ads on your website. Accurate real-time website statistics with detailed visitor tracking and analysis."