Alexa and Quantcast (and their alternatives) offer limited web statistics for free. For example, Alexa shows one-year data on a website's rank in Alexa's top. You can also download their million of most visited websites.
As for visitors, comScore MMX sells their estimates of the audience major web serivces have.
Is there any open alternatives to these sources? Those that have traffic estimates for websites, perhaps with demographics or geography?