MySQL - The world's most popular open source database
DynamoDB - Amazon DynamoDB is a fast and flexible NoSQL database service for all applications that need consistent, single-digit millisecond latency at any scale. It is a fully managed cloud database and supports both document and key-value store models.
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Datomic - The fully transactional, cloud-ready, distributed database
SQLite - SQLite Home Page
MarkLogic Server - MarkLogic Server is a multi-model database that has both NoSQL and trusted enterprise data management capabilities.