If you have seen the Netflix movie recommendations and wondered what it took to make it happen or maybe participated in the Netflix Prize, this Gigaom article discusses how Netflix spends $150 million on content recommendations every year.
Instead of each site you visit having its own recommendation algorithms, Inveni has come up with a different approach. They allow you to create an Inveni profile and then share it with sites and advertisers as you like. They are starting with movie and tv recommendations
Here is the Techcrunch article and a video.
Amazon has been recommending products based on past purchase and browse data for some time. More recently, Facebook has been suggesting users who you may want to add as friends. Now twitter is adding a suggestions for you feature based on the people you follow. A few more details from Techcrunch.