Deeply scalable Document based DBMS. It's the basic engine of all the Orient products. It can work in schema-less mode, schema-full or a mix of both. Supports advanced features, such as ACID transactions, indexing, fluent and SQL-like queries. It handles natively JSON and XML documents.
Main features:

• Extremely light: about 1Mb for the full server.
• Runs on any platform: All the engine is 100% pure Java and can run on Windows and Linux and any system that supports the Java 5+ technology (The Orient Server component needs Java 6+).
• Super fast: On common hardware* stores up to 150.000 (one-hundred-fifty-thousands) records per second, 13 billions per day.
• Transactional: Compliants with ACID tests, supports Optimistic (MVCC) and Pessimistic transaction modes
• Local mode: Direct access to the database bypassing the Server. Perfect for scenarios where the database is embedded.
• Easy Java APIs: Learn how to use it in 15 minutes.

Example of usage:

// OPEN THE DATABASE
ODatabaseDocumentTx db = new ODatabaseDocumentTx("remote:localhost/petshop").open("admin", "admin");

// CREATE A NEW DOCUMENT AND FILL IT
ODocument doc = new ODocument(db, "Person");
doc.field( "name", "Luke" );
doc.field( "surname", "Skywalker" );
doc.field( "city", new ODocument(db, "City").field("name","Rome").field("country", "Italy") );

// SAVE THE DOCUMENT
doc.save();

// QUERY THE DOCUMENT
List <ODocument> result = database.query(
  new OSQLSynchQuery ("select * from person where city.name = 'Rome'")).execute();

// PRINT THE RESULT SET
for( ODocument d : result ){
  System.out.println("Person: " + d.field( "name" ) + d.field( "surname" ));
}

db.close();

* Tested on a DELL Notebook model XPS M1530 with Intel(r) Core Duo T7700 2.40Ghz, 3 GB RAM and HD 5.400rpm