NoSQL

NoSQL is a movement promoting a loosely defined class of non-relational data stores that break with a long history of relational databases and ACID guarantees. These data stores may not require fixed table schemas, usually avoid join operations and typically scale horizontally. Academics and papers typically refer to these databases as structured storage.
This is the Next Generation Database mostly address some of the points: being non-relational, distributed, open-source and horizontal scalable. The original intention has been modern web-scale databases. The movement began early 2009 and is growing rapidly. Often more characteristics apply as: schema-free, replication support, easy API, eventually consistency, and more.
NoSQL systems often provide weak consistency guarantees such as eventual consistency and transactions restricted to single data items, even though one can impose full ACID guarantees by adding a supplementary middleware layer. Not providing relational capabilities makes it a lot easier to scale data storage by not having to pay the costs associated with relational guarantees that may not be needed by the project that uses the database.
Several NoSQL systems employ a distributed architecture, with the data being held in a redundant manner on several servers, often using a distributed hash table. In this way, the system can be scaled up easily by adding more servers, and failure of a server can be tolerated.
Some NoSQL advocates promote very simple interfaces such as associative arrays or key-value pairs. Other systems, such as native XML databases, promote support of the XQuery standard.
Below listed the NoSQL implementations that AMT is familiar with.

CouchDB
Apache CouchDB is a document-oriented database that can be queried and indexed in a MapReduce fashion using JavaScript. CouchDB also offers incremental replication with bi-directional conflict detection and resolution.

Riak
Riak is a Dynamo-inspired key/value store that scales predictably and easily. Riak also simplifies development by giving developers the ability to quickly prototype, test, and deploy their applications.

MongoDB
MongoDB is a scalable, high-performance, open source, dynamic-schema, document-oriented database. Written in C++. MongoDB bridges the gap between key-value stores and traditional RDBMS systems.

Cassandra
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 was open sourced by Facebook in 2008, and is now developed by Apache committers and contributors from many companies.