The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model. Cassandra is suitable for applications that can't afford to lose data. Data is automatically replicated to multiple nodes for fault-tolerance. 

 Cassandra provides support for Scale out, load balancing, cluster growth, Flexible schema, Key-oriented queries and CAP theorem (Consistency, Availability, Partition tolerance). It is in use at Digg, Facebook, Twitter, Reddit, Rackspace, Cloudkick, Cisco and more companies.

The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model. Cassandra is suitable for applications that can't afford to lose data. Data is automatically replicated to multiple nodes for fault-tolerance.

Cassandra - Scalable Distributed Database

HBase provides support to handle BigTable - billions of rows X millions of columns. It is a scalable, distributed, versioned, column-oriented store modeled after Google's Bigtable and runs on top of HDFS (Hadoop Distributed Filesystem). It features compression, in-memory operation per-column. Data could be replicated between the nodes. HBase is used in Facebook and Twitter.

HBase -  Hadoop database

Crate is an open source, highly scalable, shared-nothing distributed SQL database. Crate offers the scalability and performance of a modern No-SQL database with the power of Standard SQL. Crate’s distributed SQL query engine lets you use the same syntax that already exists in your applications or integrations, and have queries seamlessly executed across the crate cluster, including any aggregations, if needed.

Crate - The fast, scalable, easy to use SQL database with native full text search

Titan is a scalable graph database optimized for storing and querying graphs containing hundreds of billions of vertices and edges distributed across a multi-machine cluster. Titan is a transactional database that can support thousands of concurrent users executing complex graph traversals. It is a native Blueprints enabled graph database and as such, it supports the full TinkerPop stack of technologies.

Titan - Scalable Graph Database 

VoltDB was specifically designed for contemporary software applications that are pushed beyond their limits by high volume data sources. VoltDB provides the ability to capture, store and process incoming data at millions of read/write operations per second. And VoltDB’s relational model opens that data to be analyzed in real-time, using familiar Business Intelligence tools, to identify data patterns and trends, spot anomalies, or perform tracking and alerting. In addition to supporting real-time analytics, VoltDB seamlessly integrates with data warehousing and Hadoop infrastructures, feeding data to those environments for deep analytical exploration. Large database gets automatically sharded to multiple nodes, basically it supports database to scale out. VoltDB implements HA using K-Safety, a configurable, active-active durability model for high performance distributed systems. Client connectors are available in languages like Java, C++, Python, PHP, HTTP/JSON, Ruby and Erlang.

VoltDB was specifically designed for contemporary software applications that are pushed beyond their limits by high volume data sources. VoltDB provides the ability to capture, store and process incoming data at millions of read/write operations per second. And VoltDB’s relational model opens that data to be analyzed in real-time, using familiar Business Intelligence tools, to identify data patterns and trends, spot anomalies, or perform tracking and alerting.

VoltDB - Fast Scalable SQL DBMS with ACID

The Apache Accumulo sorted, distributed key/value store is a robust, scalable, high performance data storage and retrieval system. Apache Accumulo is based on Google's BigTable design and is built on top of Apache Hadoop, Zookeeper, and Thrift. Apache Accumulo features a few novel improvements on the BigTable design in the form of cell-based access control and a server-side programming mechanism that can modify key/value pairs at various points in the data management process.

Apache Accumulo - Key Value Store based on Google BigTable

Coherence CE (Community Edition) is a free and open source edition of Oracle Coherence, first and market-leading in-memory data grid. It has been used by hundreds of customers across many industries to power some of the mission critical systems you use every day. Coherence clusters can easily scale to hundreds of members (JVMs), all of which can both store and process the data. It can optionally persist data to disk (either local or shared), which prevents data loss even in the case of complete cluster outage.
 
Coherence is fast. Like, sub-millisecond fast. Each member of the cluster is aware of where each piece of data resides and can access it using a single network call. There are no connections to acquire or release, and no distributed transactions to commit or roll back.

Coherence CE (Community Edition) is a free and open source edition of Oracle Coherence, first and market-leading in-memory data grid. It has been used by hundreds of customers across many industries to power some of the mission critical systems you use every day. Coherence clusters can easily scale to hundreds of members (JVMs), all of which can both store and process the data. It can optionally persist data to disk (either local or shared), which prevents data loss even in the case of complete cluster outage.

Coherence CE (Community Edition) is a free and open source edition of Oracle Coherence, first and market-leading in-memory data grid

Sensei is a distributed data system that was built to support many product initiatives at LinkedIn, including the real-time faceted search in <A HREF="http://www.linkedin.com/signal/" target="_blank">LinkedIn Signal</A> and the news feed and tabs on the Homepage. Sensei is both a search engine and a database. It is designed to query and navigate through documents that consist of unstructured text and well-formed and structured metadata. 
 It supports Fast realtime updates, Complex query language and REST/JSON api, Hadoop integration and lot more.

Sensei is a distributed data system that was built to support many product initiatives at LinkedIn, including the real-time faceted search in LinkedIn Signal and the news feed and tabs on the Homepage. Sensei is both a search engine and a database. It is designed to query and navigate through documents that consist of unstructured text and  well-formed and structured metadata. Sensei is both a search engine and a database.

SenseiDB - Distributed, Realtime, Semi-Structured Database from LinkedIn

Cloudata is Distributed Large scale Structured Data Storage, and open source project implementing Google's Bigtable. It's DBMS(Database Management System), but not Relational DBMS. It can store more than Peta bytes.

Cloudata - Structured Data Storage implementing Google's Bigtable.

ConcourseDB is a distributed self-tuning database with automatic indexing, version control and ACID transactions. ConcourseDB provides a more intuitive approach to data management that is easy to deploy, access and scale while maintaining the strong consistency of traditional database systems.

ConcourseDB  - Self-tuning database designed for both transactions and ad hoc analytics across time

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Cassandra - Scalable Distributed Database

HBase - Hadoop database