RethinkDB

RethinkDB is the first open-source scalable database for the realtime web. It exposes an exciting new access model -- instead of polling for changes, the developer can tell the database to continuously push updated query results to applications in realtime. RethinkDB allows developer to build scalable realtime apps in a fraction of the time with fewer engineering resources.

RethinkDB features a pleasant and powerful query language that has useful queries like table joins and groupBy, a highly parallelized architecture, and web tools for managing clusters.