Apache Hama is my main hobby. I do a lot of contribution to it, since I think it is a valuable framework on top of Hadoop. Since Fall 2011, I am committer and PPMC member trying to push Hama to a Top-Level project at ASF. I'm doing various kinds of tasks for Hama, the most fun ones are API- and performance improvements.
I contributed the major examples of "Single Source Shortest Path" (short: SSSP) and Pagerank, I also ported Hama on top of YARN (Hadoop 23.x).