RethinkDB is an open-source, scalable JSON database built for real-time applications that allows data to be continuously pushed to clients in real-time instead of polling for changes like traditional databases. It supports installation on Ubuntu, OS X, CentOS and Debian and programmatic access through JavaScript, Ruby, and Python drivers. Queries can be run from the command line or through RethinkDB's own declarative query language called Reql. RethinkDB also supports features like indexing, aggregation, transactions, and multi-master replication across multiple machines.