An example recommendation engine that is open source (AGPLv3-licensed) has been published by Filmaster.com recently. It's written in C++ and uses best practices from the white papers produced as part of the Netflix challange. An article about it can be found at: http://polishlinux.org/gnu/open-source-film-recommendation-engine/
and the code is here: http://bitbucket.org/filmaster/filmaster-test/src/tip/count_recommendations.cpp
