Database
C / CPP
Aerospike Database Server – flash-optimized, in-memory, nosql database
Free & open source, high-performance, distributed memory object caching system
MySQL fork maintained and used at Twitter https://github.com/twitter/mysql/wiki
Redis is an in-memory database that persists on disk. The data model is key-value, but many different kind of values are supported: Strings, Lists, Sets, Sorted Sets, Hashes website
An open-source distributed JSON document database with a pleasant and powerful query language. http://www.rethinkdb.com
TokuDB is a high-performance, write optimized, compressing, transactional storage engine for MySQL and MariaDB
Javascript
MongoDB is a document database that provides high performance, high availability, and easy scalability. Documents (objects) map nicely to programming language data types. Embedded documents and arrays reduce need for joins. Dynamic schema makes polymorphism easier. [website] (https://www.mongodb.org/)
dot-net
Go-lang
An in-memory key:value store/cache (similar to Memcached) library for Go, suitable for single-machine applications.
Groupcache is a caching and cache-filling library, intended as a replacement for memcached in many cases.
JAVA
Cassandra is a partitioned row store. Rows are organized into tables with a required primary key.
LMDB for Java, which is a very fast embedded key/value store with full ACID semantics.
MapDB provides concurrent Maps, Sets and Queues backed by disk storage or off-heap-memory. It is a fast and easy to use embedded Java database engine.http://www.mapdb.org