C/C++

• http://sphia.org/ a Key/Value Store, Written in C and BSD-Licensed.
• https://code.google.com/p/leveldb/ Written in C++, also BSD-Licensed
• http://luxio.sourceforge.net/ Written in C++, LGPL, faster than Berkeley DB
• http://symas.com/mdb/ Written in C, License is OpenLDAP Public License (BSD-Like)
• https://github.com/davidar/sdbm
  Written in C, Public Domain, Reimplementation of “ndbm”
• http://repetae.net/computer/vsdb/ Written in C, License unknown, Experimental
• https://github.com/stevedekorte/skipdb Written in C, BSD-Licensed, Key-value Store using Skip-List indexes
• http://unqlite.org/
  written in C/C++, Embedded NoSQL Database Engine, BSD-Licensed, Similar to SQLite.
• http://hamsterdb.com/
  Written in C/C++, Embedded NoSQL Database Engine, Apache License 2.0.

Java

• https://github.com/dain/leveldb A Java port of LevelDB, Apache License 2.0
• https://github.com/jingwei/krati A former LinkedIn Project, Apache License 2.0
• http://www.mapdb.org/ An Embedded Java Database, Apache License 2.0, supports Key-Value Storage, very efficient Object serialisation
• https://github.com/OpenHFT/Chronicle-Map, LGPL v3, embedded pure Java key-value store, faster than Berkeley DB
• https://github.com/jetbrains/xodus, Apache License 2.0, a Java transactional schema-less embedded database, faster than Berkeley DB
• https://github.com/lmdbjava/lmdbjava, JNI binding for LMDB, Apache License 2.0