MUSCLE (Multi User Server Client Linking Environment) is an N-way messaging server and networking API. It includes client-side networking APIs for various languages, including C, C++, C#, Delphi, Java, and Python. MUSCLE lets programs communicate over a network via streams of serialized Message objects. The included server program ("muscled") lets its clients message each other and store information in its server-side hierarchical database. The database supports flexible queries via hierarchical wildcarding, and "live" updates via a subscription mechanism.
HSQLDB (HyperSQL DataBase) is an SQL relational database engine written in Java. It supports nearly full ANSI-92 SQL and SQL:2008 enhancements. It is a small, fast multithreaded, and transactional database engine which offers in-memory and disk-based tables and supports embedded and server modes.
KInterbasDB is a Python extension package that implements Python Database API 2.0-compliant support for the open source relational database Firebird and some versions of its proprietary cousin Borland Interbase. In addition to the minimal feature set of the standard Python DB API, KInterbasDB also exposes nearly the entire native client API of the database engine.
SibylSQL is a portable, full-fledged, next-generation, client-server, multi-threaded, object-relational database system. The primary technical contribution to the database community is the inclusion of the SQL Dialect Interface (SDI), originally written for the NightStar NEXTGRES database. The SDI allows a DBA to grant users or applications the ability to translate vendor-specific SQL such as Oracle, SAP DB, MSSQL, and PostgreSQL, into a native SibylSQL parse tree via dynamically loadable objects at run-time.
PostgreSQL-relay can be used to as a single point of origin for a number of databases. Instead of having to remember (or modify in case of changes) all the names of databases, on which machines and on which ports they are running, you only need to remember one machine and the name of the database. It will then forward the connection to the proper database on the proper port of the proper server.
TwinDB is an embedded key/value database based on BerkeleyDB. It basically adds security from database corruption on top of the non-transactional version of BerkeleyDB. It accomplishes this by maintaining twin BerkeleyDB databases and using "lock" files to indicate when a set of writes are in progress. Performance is still good (tested at ~60K writes/sec and 300K reads/sec). It was written because the transactional version of BerkeleyDB is expensive.
SQLite/Ruby is a set of bindings to allow the SQLite (v 2.x) database to be used from Ruby scripts. It provides an object-oriented interface to all major functions of the SQLite API, including the create_function and create_aggregate functions. It also supports bind parameters, transactions, internal and external iterators, type translation in queries, and easy access to various pragmas.
Slony-I is a "master to multiple slaves" replication system supporting cascading and slave promotion. The big picture for the development of Slony-I is as a master-slave system that includes all features and capabilities needed to replicate large databases to a reasonably limited number of slave systems. Reasonable, in this context, is probably no more than a few dozen servers. If the number of servers grows beyond that, the cost of communications becomes prohibitively high.