PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Datomic - The fully transactional, cloud-ready, distributed database
SQLite - SQLite Home Page
Datahike - A durable datalog database adaptable for distribution.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
Matisse - Matisse is a post-relational SQL database.