Presto is an amazing open source MPP engine. It has excellent architecture and an active community. We ( http://www.treasuredata.com) use it extensively as one of of our big data SaaS query engines (also we contribute back a lot to Presto).
Qubole offers Presto as a Service with a free trial (http://www.qubole.com) with a bunch of interesting enhancements for the cloud. We've found Presto to be a great engine for realtime queries over large datasets.