nf1db is an in memory database engine with SQL and JDBC interfaces. It supports transactions, commands from the SQL-2003 standard, all major data types, a number of functions, default and null values, auto-increments, constraints, primary and foreign keys, set operations, implicit and explicit joins, nested SELECT statements, quantity limits in SELECT statements, and much more.
Bitsy is a small, fast, embeddable, durable in-memory graph database that implements the Blueprints API. It is an ACID database that recovers cleanly from power failures and crashes. Designed for multi-threaded OLTP applications, Bitsy implements optimistic concurrency control and stores data in human-readable JSON-encoded text files. Common management tasks such as online backups can be done over JMX. The speed and simplicity of Bitsy relies on three design principles: No Seek, No Socket, NoSQL.