Another thing: a site you go to can scan ALL your cookies looking to see what sites interest you. You then can end up plugged into a different set of banners that more fit the profile of what you are apt to buy. And you might start received related unsolicited email that fits your market profile. If you have visited a lot of automotive sites, you might see banners about cars even when you're at a non-automotive site. Hey, this would explain why my boss gets lots of spam related to porn sites...