Quantcast is an interesting "open internet ratings service". Much like Alexaholic is a poor man’s comscore, Quantcast provides demographic information on various websites pulled from their affiliated partners. It will give you a quick overview, though the accuracy is going to highly variable depending upon the panels and the site. That said, it is interesting to get a rough view of who is getting drawn to a specific site. Serious analysis will obviously go to comScore, Nielsen and such for reliable stats.