comparison:

- http://www.quantcast.com/macrumors.com#traffic (measured)

- http://siteanalytics.compete.com/macrumors.com/

Edit: Apparently saying something factual gets downvotes at HN nowadays, so let me add some sources.

http://www.compete.com/resources/methodology/

http://aws.amazon.com/awis/faqs/#Where_does_Alexa_get_the_tr...

http://www.quora.com/Where-does-Compete-get-its-data-from

http://wackao.com/which-is-more-accurate-alexa-rank-or-compe...

http://www.bnet.com/blog/technology-business/comscore-money-...

First hand experience