Keyspace is a replicated key-value database. Keyspace is set apart from other key-value database because it uses so-called consistent replication, which means that all nodes see the database going through the same states. In other words, no split-brain scenario can occur, where different nodes see different versions of the database.

WhiteDB is a lightweight NoSQL database library written in C, operating fully in main memory. There is no server process. Data is read and written directly from/to shared memory, no sockets are used between WhiteDB and the application program.

ClickHouse is an open-source column-oriented database management system that allows generating analytical data reports in real time. ClickHouse manages extremely large volumes of data in a stable and sustainable manner. It currently powers Yandex.Metrica, world’s second largest web analytics platform, with over 13 trillion database records and over 20 billion events a day, generating customized reports on-the-fly, directly from non-aggregated data. This system was successfully implemented at CERN’s LHCb experiment to store and process metadata on 10bn events with over 1000 attributes per event registered in 2011.

hamsterdb Embedded Storage is a lightweight embedded "NoSQL" key-value store. It is in development for more than five years and concentrates on ease of use, high performance, stability and scalability.

NoSQL with MySQL in Ruby

MemcacheDB is a distributed key-value storage system designed for persistent. It is NOT a cache solution, but a persistent storage engine for fast and reliable key-value based object storage and retrieval. It conforms to memcache protocol(not completed, see below), so any memcached client can have connectivity with it. MemcacheDB uses Berkeley DB as a storing backend, so lots of features including transaction and replication are supported.

Titan is a highly scalable graph database optimized for storing and querying large graphs with billions of vertices and edges distributed across a multi-machine cluster. Titan is a transactional database that can support thousands of concurrent users.

GoshawkDB is an open source distributed, transactional, fault-tolerant object store, which puts its semantics and guarantees front and center.

Membase Server is a simple, fast, elastic NoSQL database. As a distributed key-value database, it is optimized for the data management needs of interactive web applications and used to power some of the world's busiest web applications, such as those at Zynga. It scales out and supports live cluster topology changes while continuing to service data operations without performance degradation. Even Membase software upgrades can be performed with zero application downtime. Membase Server is fast, and consistently so, sustaining low-latency and high-throughput random read and write operations within a very narrow performance band across the entire scaling spectrum – from one to hundreds of servers.

MongoDB bridges the gap between key-value stores (which are fast and highly scalable) and traditional RDBMS systems (which provide rich queries and deep functionality). MongoDB (from "humongous") is a scalable, high-performance, open source, document-oriented database.