PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
MySQL - The world's most popular open source database
Datahike - A durable datalog database adaptable for distribution.
MariaDB - An enhanced, drop-in replacement for MySQL
MarkLogic Server - MarkLogic Server is a multi-model database that has both NoSQL and trusted enterprise data management capabilities.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.