Most online recommendation engines use collaborative filtering based on purchase or social networks.

FORBES: The Trouble With Finding Books Online - And A Few Solutions

One of the most well-established methods for doing this is collaborative filtering--the analysis of user preferences and history--to make recommendations.

FORBES: Magazine Article

Tihnk about how the e-commerce experience leverages clickstreams, cookies, implicit and explicit personalization and collaborative filtering to drive a personalized experience for the visitor.

FORBES: For Mobile Commerce: The Year Of Convergence And Context

The app relies on a combination of collaborative filtering and the parsing of data about the ingredients, cooking methods and other variables that comprise a dish into linked and relational data.

FORBES: Food Genius Digitizes the Dish to Build Netflix For Foodies

But through collaborative filtering, you're going to find it.

CNN: Mobile screens seek portable content

My guess is that if it is done right, collaborative filtering of content can expose the long tail better than the current cable bundle, at which point Apple (or some other company) would really have something to talk to the cable companies about.

FORBES: Tim Cook Again Expresses Intense Interest In TV Market, But Mutes The Real Issues

Books lend themselves particularly well to an Internet-selling tool known as "collaborative filtering": Amazon and other Web vendors ask you to name some of your favorite products -- in Amazon's case, the books, authors, or genres you like best -- and then the retailers' software program tells you what other people with similar tastes liked so you can add to your shopping list.

FORBES: Does Amazon.com really matter?