Features:

• It is free to use for everybody, source code is included
• Written in Java, but also available as native executable
• JDBC and (partial) ODBC API
• Embedded and client/server modes
• Clustering is supported
• A web client is included
• Fast database engine
• Free, source code is included
• Supports standard SQL
• Disk based or in-memory databases
• Browser base Console application
• Wide range of data types including large objects (BLOB/CLOB)
• Strong security features (SSL, users, roles, encrypted files)
• Multiple connections, table level locking
• Cost based optimizer, using a genetic algorithm for complex queries
• Transaction support (serializable transaction isolation)
• Zero-administration, automatic recovery
• Large resultset, external sorting
• Scrollable and updatable resultset support
• Read only views and inline views
• Inner and outer joins, subqueries
• Many functions already built-in
• Compatibility modes for HSQLDB, MySQL and PostgreSQL
• Small footprint (smaller than 1 MB)
• Uses a small number of database files
• Can be compiled to native code using GCJ
• Primary keys, multiple index types (b-tree, linear hash, tree, hash)
• ORDER BY, GROUP BY, HAVING, UNION, LIMIT
• The database can generate SQL script files
• Support for linked tables, and a built-in virtual ´range´ table
• Automatic re-compilation of prepared statements
• Sophisticated trace options
• Low memory requirements
• Standard handling of NULL values
• Multilanguage console application (at this time, English and German)
• Much more

Requires Java Runtime Environment. Get It Here.