Relative to Dan's recent mention of SOM as possible new project, here are slides from KDD Cup 2012 in which Stephen Rendle describes how he did using a very straightforward implementation of Factorization Machines [1,2].
FMs are interesting in the context of Mahout because they can be used in a wide variety of settings including recommendation and targeting and because they have very good performance on a number of tasks. I should mention that Robin was the one who first mentioned FMs to me. The KDD 2012 competition [3] is of interest in any case because it provides a large amount of realistic data for commercially important problems. [1] https://kaggle2.blob.core.windows.net/competitions/kddcup2012/2748/media/RendleSlides.pdf [2] https://kaggle2.blob.core.windows.net/competitions/kddcup2012/2748/media/Rendle.pdf [3] http://www.kddcup2012.org/
