Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
Aerospike is a high-performing NoSQL database supporting high transaction volumes with low latency.
High-performance, distributed memory object caching system
Clustering and highly scalable data distribution platform for Java
Database for real-time transactions and analytics.
In-memory relational DBMS capable of supporting millions of database operations per second
High performance, in-memory storage engine for time series data (by Facebook)