PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Amazon Aurora - MySQL and PostgreSQL-compatible relational database built for the cloud. Performance and availability of commercial-grade databases at 1/10th the cost.
SQLite - SQLite Home Page
Citus Data - Worry-free Postgres. Built to scale out, Citus distributes data & queries across nodes so your database can scale and your queries are fast. Available as a database as a service, as enterprise software, & as open source.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
InfluxData - Scalable datastore for metrics, events, and real-time analytics.