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.
SQLiteDBMS is a database management server for SQLite. It allows an sqlite3 process to be accessed via a TCP/IP network. It provides Extended SQL, SSL, basic authentication, query caching, WebDAV, access control, and replication. It includes a simple Web server that takes HTTP requests and sends XML responses.
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.
The Skeleton Engine is a starting point for creating a MySQL Storage Engine. It comes with everything you need to write a pluggable engine. Autoconf, an initial framework, and scripting tools are all provided. The Skeleton Engine has been the starting point for the PBXT, Nitro, Memcache, AWS, HTTP, and many other engines.
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.