They might be relatively small, but http://www.grouplens.org/node/12 http://www.grouplens.org/node/12 has some interesting datasets that can be used to experime

They might be relatively small, but http://www.grouplens.org/node/12 http://www.grouplens.org/node/12 has some interesting datasets that can be used to experiment with recommendation systems, e.g. book and movie reviews.