Which companies try to get to know you as an online consumer by tracking you?

Probably most companies that exist online to one extent or another. Some just track what you have bought from them in the past. Others want to know what other sites you have visited, where you live, whether you have kids, and even more personal things. They want to get to know you so that they can serve you better... it isn't all bad. It just gets out of control if they sell that information to others, or if someone steals it from them.