db4o is an object database for Java and all .NET environments. Its features include ACID transactions, automatic class schema recognition, query-by-example, a S.O.D.A. object querying API, native queries, LINQ support on .NET, callback methods, memory files, cascade-on-delete, multi-threaded access, and Java Webstart and servlet support.
PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS), much like ESRI's SDE or Oracle's Spatial extension.
hamsterdb Embedded Storage is an embedded database engine written in ANSI-C. It includes B+Trees with variable length keys and records. It supports in-memory databases and endian-independent files, database cursors, multiple databases in one file, "record number" databases, and duplicate keys. hamsterdb is very fast and highly configurable. It compiles and runs on Unix platforms, Linux, Microsoft Windows, and Windows CE.
SimpleDB/dev is a local instance of Amazon's SimpleDB. It runs as a server on your local machine and replicates the SimpleDB REST API as specified in Amazon's technical documentation. It implements every SimpleDB query-action and includes a large suite of tests, created from the query examples provided by Amazon.
jpt is a generic database toolkit. The package's main component is the libjpt library, which can be easily linked into C++ and C programs. A jpt database consists of a single sparse table, with an arbitrary number of rows and columns. The programming interface is simple, and the data structures are designed for fast random inserts and lookups.