A simple item based algorithm which has been reported to work quite well is Slope One. The advantages are that it is easy to implement, can be updated on the f

A simple item based algorithm which has been reported to work quite well is Slope One. The advantages are that it is easy to implement, can be updated on the fly, and it works well (enough) for sparse data sets. http://www.daniel-lemire.com/fr/abstracts/SDM2005.html http://www.daniel-lemire.com/fr/abstracts/SDM2005.html There's also examples using python, java, and PHP/SQL.