I am using the account of Histats.com. As it is one of the best stats calculation website available on internet for free. If I compare it with other websites in this category then they charge few amount for the same features which Histat is providing for free.
Now, the income source of Histats is banner advertisements provided by Buy-Sell Ads. Through the help of website content and the visitors quality it was so easy to get approved for Buy Sell Ads for this website.
I still remember the time, when I created my account on it. This website was having Google Adsense advertisement. After using Adsense for few months, this website decided to become the publisher of BSA. Through the stats which are available on the BSA website on http://buysellads.com/buy/detail/29222.
As you will be able to see the stat which is mentioned on the data page as well as on the above screenshot. It has been captured on 30th November, 2011. As on this date the monthly income of Histats.com is generating as :
$ 500 X 3 = $ 1500
(+) $ 390 X 5 = $ 1950
(+) $ 380 X 5 = $ 1900
(+) $ 290 X 6 = $ 1740
Total = $ 7090
The total amount varies around $ 7000 per month. Hence, the website comes in the category of 6 figure monthly income. All the ad spots of the website has been sold out of this website with great rates.