Hitwise (owned by Experian) buy web browsing logs from internet service providers. They use these anonymous logs to build reports showing how users brows the web. Want to see which sites people visit immediately before they visit your biggest competitor? Want to see roughly how much traffic a site gets, and how that splits between paid, organic, and referrals? Hitwise offers all of this for a sample of users. It’s not perfect from a user interface point of view, and the data is a sample (and therefore patchy particularly on smaller site data), but it’s still incredibly useful in some contexts. For example if you’re working on a site with a big, big direct competitor, it’s so useful to be able to reverse engineer where their traffic comes from.