You are required to place a 468 x 60 pixel banner advert on your page - that isn't as bad as it sounds because they provide an excellent service and quality adverts are delivered. Features of this tracker include hits per day, referrers, operating systems, domains and a lot more besides. The only thing that stops it getting 10 is your stats can be viewed by anyone. It tracks an extensive amount of information, including visitor totals and averages, top referrers, browser types, java script status, keywords and even countries. A small button is placed on your page when you copy and paste a small HTML code into your site.
Invisible counters are available but at a cost.
Live - Seen running on the left navigation bar, this tracker is probably the best. It has every function you could ever want, tracks last 100 referrers, most requested pages, search phrases, countries and more. They provide detailed statistical reports, which include general traffic levels, times of visits, countries, referral channels, browser types, ISP's and even individual pages. Hitbox - Hitbox provides well over 100 different stats about your visitors for free. Hitbox also tells you browser types, rush hours, referrers, countries, languages, IP addresses and lots more besides.