open-source Recommendation Engine

Since you haven't specified any constrains like language or platform, here a few projects that come to my mind:

• Mahout, part of the Hadoop project.
• Taste, which is now part of Mahout (see above)
• Duine, more of an prediction engine, but could be used as an recommender system.
• Cofi, Java based
• SUGGEST

There is also a wealth of high quality research papers with many good ideas (like the ones who won the Netflix price) out there. For a starting point, just look over here.