They also provide information relevant to advertisers such as demographic breakdown including: gender, age, ethnicity, income level. Global traffic vs US is also useful. The data can be quite accurate for sites that have included the Quantcast code but, even those that don't are estimated by panel participants and ISP data.
It's interesting to see what sites make up the top 100. There are the usual suspects but, some names you have probably never heard of. If you create an account they also offer advanced search features which will find similar sites to ones that you like, demographic information, or by numerous other criteria.
