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?
To all my available knowledge having done significant research on digital business models for a number of years, no, no there is not much of a comprehensive, free source for this type of enterprise data, as it requires expensive resources to keep updated and maintained, and even then isn't scientific or, as you put it, is merely an estimate.
I know that's not a helpful answer necessarily, but it's reasserting the need for an open alternative and -- to my knowledge -- letting you know that I personally have not come upon one.
