I have been playing with www.quantcast.com for a couple of weeks. It seems they are trying to be similar to Alexa, but it requires the site owner to put some JavaScript on every page. The results are demographics about your site visitors - for instance, you can see info about my site here: http://www.quantcast.com/deepmarket.com - apparently, where childless males making over 100K rule! So far it is free - which is good, I would not pay for this level of detail. It may be interesting for site owners to use for selling advertising. I guess I need to try Google Analytics - does it have any of these demographic features?