The new Recommendation Module for Drupal is a "collaborative filtering" engine. This means that the module predicts interesting nodes, according to your personal tastes.

The innovative principle is based on "automated user-based collaborative filtering". Check, for example, Amazon.com's "Customers who bought this book also..." or the MovieLens university research project.

Think of it as a "word-of-mouth" system, where friends would recommend you interesting nodes.
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As an example, let's say that you rate a couple of nodes (as "Not Recommended", "Recommended", or "Highly Recommended"). After rating a few nodes, the module will match your profile with the other users on the website. Now knowing who your "friends" are, the module can list the nodes that you might be interested in.

The module can be downloaded at http://hcilib.sourceforge.net/kyvinh/recommendation.zip and is compatible with Drupal 4.1RC. For developers, the package includes a DEVELOP file that summarizes the code.

Additional pointers:Research papers and links to working systems like the great MovieLens or the pathetic TV Recommender.