After Google recently got bad grades for the way they handle privacy, I have to add a good quote, that I've found a while ago:
"At the Open Data 2007 conference in New York today, David Cancel, the CEO of Compete Inc. revealed that ISPs happily sell clickstream data -- and that it's a big business. They don't sell your name -- just your clicks -- but the clicks are tied to you as a specific user (User 1, User 2, etc.)....How much are your clicks worth? About 40 cents a month per user (per customer)... and the Compete CEO estimates that there are 10-12 big buyers of this data. In other words, your ISP is probably making about $5 a month ($60 a year) off your clickstreams"