One of the interesting questions is: "what are the most useful machine learning algorithms?".
I did a little survey by looking at the Mahout user mailing list and counting occurrences of keywords. The results I got are shown in the plot above.
It seems that matrix factorization (SVD) is the most widely used algorithm, and then K-means. We have just implemented SVD as a part of the GraphLab Collaborative Filtering library. Anyone who wants to beta test it is welcome!