Riak is an open-source licensed distributed key-value- and document-store developed by Basho Inc. Riak has all the features of state-of-the-art distributed KV-stores ranging from an HTTP/REST-based API to an integrated MapReduce framework and pluggable storage-engines.

Riak's unique contribution is its flexible query facilities. It's datastore can be queried by a simple description of a graph traversal process which is transformed into a chain of MapReduce steps at query-time. The result is a system with flexible data-extraction, -filtering and -processing possibilities that go beyond "traditional" MapReduce-enabled stores.

Riak's code base is small and clean such that is is easy to understand and extend its implementation. By this, it is both a great foundation for building own systems on top of Riak and a great learning resource for all things NoSQL.