This FREE tool will tell you exactly the demographic of the people visiting a website. I do know you have to have a certain amount of traffic for it to populate the data for you, but if you are in the business of website redesign for higher traffic websites, what an awesome tool. The website for this is
Quantcast and it is really a very interesting look into your target market.

For instance, I entered realtor.org in and saw the complete demographics for the people visiting that site. This will help me in targeting my audience because it's mostly real estate professionals visiting the site, which is my target audience. Hope you all find this tool as helpful and interesting as I do!