Apache Phoenix: http://phoenix.apache.org/ is another SQL engine on top of Hbase/Hadoop. If you have an existing HBase cluster, Phoenix is a much easier way to put a SQL layer on top by simply mapping existing hbase tables to Phoenix tables. The icing is the ability to use JDBC tooling to query the system (Squirrel, DbVisualizer).
There's also Presto from fb, and Apache Drill. Phoenix seems to be on top of Hbase, which is more funkier that plain HDFS + SQL engine.
Besides, right now there's a huge need for analytical engines on top of HDFS (data lake or whatever you call it) - that's why there are all of these: (shamelessly stolen from an email from drill users):