StatCounter

Invisible or visible tracker which can be used to track unique visitors and pageloads. Provides sophisticated visitor data including unique and returning visitors, popular pages, entry and exit points, keyword analysis, visitor paths and visit lengths, and more. Online demo available. No ads, and free for sites with less than 9,000 impressions per day.

Webstats4U

Free web counter service for measuring traffic to a single page, which uses an 18x18 pixel tracking graphic. Stats include page view counts for the last month and basic info about your visitors' countries of origin. All counter data can be viewed by anyone. The service's homepage displays the top traffic sites. The service also launches popup and other ads.

Worth a look if you're willing to pay...