MySQL is an open-source relational database management system. It is a popular choice of database for use in web applications, and is a central component of the widely used LAMP open-source web application software stack.

PostgreSQL, often simply Postgres, is an ORDBMS with an emphasis on extensibility and standards-compliance. It can handle workloads ranging from small single-machine applications to large Internet-facing applications with many concurrent users.

MongoDB is a document-oriented database that is classified as a NoSQL database. It eschews the traditional table-based relational database structure in favor of JSON-like documents with dynamic schemas.

Riak is a distributed NoSQL key-value data store that offers high availability, fault tolerance, operational simplicity, and scalability. In addition to the open-source version, it comes in a supported enterprise version and a cloud storage version.

Elastic is a NoSQL database and search server based on Lucene. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents.

Redis is an open source, in-memory data structure store, used as database, cache and message broker. It supports data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs and geospatial indexes with radius queries.

Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering. It is intended for use in speeding up dynamic web applications by alleviating database load.

Apache Cassandra is an open source distributed database management system designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure.

CouchDB is a database that uses JSON for documents, JavaScript for MapReduce indexes and regular HTTP for its API.