Apache CouchDB
CouchDB (sometimes "CouchDb") is an open-source distributed document-centric non-relational database written in Erlang. It is designed for extreme scalability, and is easily deployed to multi-core or multi-server clusters.
CouchDB is unique among popular open source databases in that it is *not* a Relational Database Management System - instead of storing data in rows and columns, the database manages a collection of JSON documents (early versions of CouchDB used XML). As a result, it is often compared with column-oriented datastores like Google's BigTable; however, CouchDB is not a column-oriented store, since the documents in a collection need not share a schema. Aggregate functions and filters are computed in parallel as in MapReduce, rather then at query time (although queries may introduce new aggregate functions, which execute on the existing documents in parallel). Currently the preferred query language is Javascript, although the database is easily modified to process functions written in PHP, Ruby, and other mainstream languages.