eXTReMe Tracking provides some of the most detailed and useful tracking statistics I've seen from a free service. Apart from tracking and counting visitors, the eXTReMe system notes exactly where they came from, divides the sources into categories (search engines, websites, email links etc) and compares them (ie # of visitors originating from Google compared to the number arriving via Bing). It even notes the most common keywords used to find your site. This is a great service for anyone interested in promoting their site.