Here's this from an NPR story last August: ""The one site that installed the most was Dictionary.com. A visit to Dictionary.com resulted in 234 trackers being installed on our test computer, and only 11 of those were installed by Dictionary.com. Some tracking devices are completely innocuous. A cookie, or some type of tracker that remembers your password, [can be innocuous]. So if you ask a website to remember your login, that can be stored on a cookie. There are tracking devices that are useful to you as a Web browser. And those tend to be the ones that are installed by the website that you actually have a relationship with, not the ones that you've never heard of before that are sort of secretly lurking behind the scenes. So on Dictionary.com, the vast majority of the trackers (200 out of 234) were installed by companies that the person visiting the site probably had never heard of.""
Read more scary stuff here.