LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values. Keys and values are arbitrary byte arrays. It supports basic operations like Put(key,value), Get(key), Delete(key). Only a single process (possibly multi-threaded) can access a particular database at a time. 

Leveldb - A fast and lightweight key/value database library by Google.

ObjectBox is a superfast object-oriented database with strong relation support. ObjectBox is embedded into your Android, Linux, macOS, or Windows app. ObjectBox is a mobile database built for objects from ground up (no ORM, no SQLite). The concise API is easy to learn and just takes a fraction of the code you need to work with SQLite. ACID properties and Multiversion Concurrency Control (MVCC) provide you with safe transactions and parallelism.

ObjectBox - Super fast database for Mobile and IoT

Neo4j is a high-performance graph engine with all the features of a mature and robust database. It is a graph database, storing data in the nodes and relationships of a graph. It includes the usual database features like ACID transactions, durable persistence, concurrency control, transaction recovery, high availability. It is massively scalable to handle graphs of several billion nodes on a single machine. It has a powerful traversal framework for high-speed traversals in the node space. It can be deployed as a full server or a very slim database embedded database. 
Neo4j's high-performance distributed cluster architecture scales with your data needs, minimizing cost and hardware while maximizing performance across connected datasets. Neo4j enforces rigorous enterprise security rules while remaining easy to deploy and manage. Protections like “row-level” security in relational databases are not enough when using graphs. Organizations need fine-grained access control for mission-critical security and privacy.

Neo4j is a high-performance graph engine with all the features of a mature and robust database. It is a graph database, storing data in the nodes and relationships of a graph. It includes the usual database features like ACID transactions, durable persistence, concurrency control, transaction recovery, high availability.

Neo4j - Graph Database

HyperGraphDB is a general purpose, open-source data storage mechanism based on a powerful knowledge management formalism known as directed hypergraphs. While a persistent memory model designed mostly for Knowledge management, Artificial Intelligence and Semantic web projects, it can also be used as an embedded object-oriented database for Java projects of all sizes. It could also be used as graph database or as (non-SQL) relational database. 

 A P2P framework for distributed processing has been implemented for replication/data partitioning algorithms as well as client-server style computing. While it falls into the general family of graph databases, it is hard to categorize HyperGraphDB as yet another database because much of its design evolves around providing the means to manage structure-rich information with arbitrary layers of complexity. 

 Source code location: <A HREF="http://code.google.com/p/hypergraphdb" target="_blank">http://code.google.com/p/hypergraphdb</A>

HyperGraphDB is a general purpose, open-source data storage mechanism based on a powerful knowledge management formalism known as directed hypergraphs. While a persistent memory model designed mostly for Knowledge management, Artificial Intelligence and Semantic web projects, it can also be used as an embedded object-oriented database for Java projects of all sizes. It could also be used as graph database or as (non-SQL) relational database.

HyperGraphDB - Database for Storing Strongly-Typed Hypergraphs

Oracle Berkeley DB provides the best open source embeddable databases allowing developers the choice of SQL, Key/Value, XML/XQuery or Java Object storage for their data model. At its core is a fast, scalable, transactional database engine with proven reliability and availability. Berkeley DB comes three versions: Berkeley DB, Berkeley DB Java Edition, and Berkeley DB XML.

Berkeley DB - Embeddable Databases to Meet Your Needs

HyperSQL supports the widest range of SQL Standard features seen in any open source database engine. It offers a small, fast database engine which offers in-memory and disk-based tables and supports embedded and server modes. 
 Its major feature include: 
 <UL>
	<LI>Java stored procedures and functions</LI>
	<LI>Java triggers</LI>
	<LI>Extensive JDBC interface support </LI>
	<LI>Persistence</LI>
	<LI>Embedded (into Java applications) and Client-Server operating modes</LI>
	<LI>Multiple databases per JVM </LI>
	<LI>ODBC support</LI>
	<LI>simple GUI database management tools</LI>
 </UL>
 Commercial support is available.

HyperSQL supports the widest range of SQL Standard features seen in any open source database engine. It offers a small, fast database engine which offers in-memory and disk-based tables and supports embedded and server modes. It is a relational database engine written in Java, with a JDBC driver, conforming to ANSI SQL:2008. It alsi supports transaction isolation, multiversion concurrency and ACID, Java stored procedures, Scalability and lot more.

HyperSQL - Java Database

STSdb is an embedded object database system (NoSQL key/value pair storage engine) designed for mission critical and real-time applications. It provides intuitive use and blazing performance. It has transaction support, Snapshots, LINQ, Custom compression and lot more. STSdb is implemented and inter-operate on Windows, Linux, BSD, UNIX, Mac OS X under the Mono platform.

STSdb - Embedded .NET Database

Apache Derby is an open source relational database implemented entirely in Java. It has a small footprint that makes it easy to embed in any Java-based application, but it also supports the more familiar client/server mode. It is based on the Java, JDBC, and SQL standards, making code developed more portable to standards-compliant databases.

Apache Derby - Relational Database implemented in Java

RocksDB is an embeddable persistent key-value store for fast storage.  RocksDB builds on LevelDB to be scalable to run on servers with many CPU cores, to efficiently use fast storage, to support IO-bound, in-memory and write-once workloads, and to be flexible to allow for innovation. It is especially suited for storing data on flash drives. It has multi-threaded compactions, making it specially suitable for storing multiple terabytes of data in a
single database.

RocksDB - A persistent key-value store for fast storage environments

UnQLite is a in-process software library which implements a self-contained, serverless, zero-configuration, transactional NoSQL database engine. UnQLite is a document store database similar to MongoDB, Redis, CouchDB etc. as well a standard Key/Value store similar to BerkeleyDB, LevelDB, etc.
 
UnQLite is an embedded NoSQL (Key/Value store and Document-store) database engine. Unlike most other NoSQL databases, UnQLite does not have a separate server process. UnQLite reads and writes directly to ordinary disk files. A complete database with multiple collections, is contained in a single disk file. The database file format is cross-platform, you can freely copy a database between 32-bit and 64-bit systems or between big-endian and little-endian architectures.

UnQLite is a in-process software library which implements a self-contained, serverless, zero-configuration, transactional NoSQL database engine. UnQLite is a document store database similar to MongoDB, Redis, CouchDB etc. as well a standard Key/Value store similar to BerkeleyDB, LevelDB, etc.

UnQLite - Transactional Embedded NoSQL Database Engine

Dqlite is a fast, embedded, persistent SQL database with Raft consensus that is perfect for fault-tolerant IoT and Edge devices. Dqlite (“distributed SQLite”) extends SQLite across a cluster of machines, with automatic failover and high-availability to keep your application running. It uses C-Raft, an optimised Raft implementation in C, to gain high-performance transactional consensus and fault tolerance while preserving SQlite’s outstanding efficiency and tiny footprint.

Dqlite - High-availability SQLite

H2 database is very fast, open source database engine. 
 It supports : 
 <UL>
	<LI>It supports SQL and JDBC standards</LI>
	<LI>Encrypted database</LI> 
	<LI>Security and Authentication</LI>
	<LI>Multiple index types (b-tree, tree, hash)</LI> 
	<LI>Contains a recovery tool that can dump the contents of the database</LI> 
	<LI>Clustering</LI>
	<LI>Full text search</LI>
	<LI>Temporary Tables</LI> 
	<LI>User-Defined Datatypes</LI>
 </UL>

H2 database is very fast, open source database engine. It supports SQL and JDBC standards. It can run in Embedded and Server mode and it has clustering support.  


H2 Database - Java based Database Engine

SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world. It is faster than popular client/server database engines. 
 
SQLite is popular choice for the database engine in cellphones, PDAs, MP3 players, set-top boxes, and other electronic gadgets. SQLite has a small code footprint, makes efficient use of memory, disk space, and disk bandwidth, is highly reliable, and requires no maintenance from a Database Administrator.

SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world. It is faster than popular client/server database engines. 

Sqlite - SQL database engine

NeDB is an Embedded persistent or in memory database for Node.js, nw.js, Electron and browsers, It is 100% JavaScript, no binary dependency, its API is a subset of MongoDB. One datastore of NeDB is equivalent to a MongoDB collection. 

NeDB - Embedded datastore for node.js

MapDB is an embedded database engine. It provides Maps and other collections backed by disk or memory storage. It offers excellent performance comparable to java collections, but is not limited by GC overhead. It is also a full database engine with storage backends, transactions, cache algorithms, expiration and many other options. MapDB provides concurrent Maps, Sets and Queues backed by disk storage or off-heap memory. 

MapDB - Embedded Database Engine

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