A Guide to Recommender Systems

4 main approached to recommendations:

• Personalized recommendation - recommend things based on the individual's past behavior
• Social recommendation - recommend things based on the past behavior of similar users
• Item recommendation - recommend things based on the item itself
• A combination of the three approaches above

Amazon: King of Recommendations

Amazon's system is very sophisticated, but at heat all of its recommendations are based on individual behavior, plus either the item itself or behavior of other people on Amazon

Google: Focus on Personalized Recommendations

http://www.readwriteweb.com/archives/recommender_systems.php