Server-Based Open Source DBMS

Here's a short survey of open source database management systems:

• MySQL - MySQL is, I think, the most popular, and best known open source DBMS. It is cross-platform. It is the most popular because historically, it has been the fastest of the open source DBMS, but it has always lagged behind in terms of ACID compliance and other features. You can access a MySQL database via many many different drivers that people have written for just about any programming language. It is also possible to access MySQL databases via ODBC (Open DataBase Connectivity) or JDBC (Java DataBase Connectivity)
• PostgreSQL - PostgreSQL has always been my favorite. I've been using it since it was called Postgres95 - before version 6. (Wikipedia has a great entry on PostgreSQL, including some history). PostgreSQL has always been ahead of MySQL in terms of ACID compliance and robustness, and still is. It lagged behind MySQL for years because of speed issues (it was much slower,) but that has changed with the newest versions, such that in fact PostgreSQL is faster and more scalable than MySQL. PostgreSQL is also cross-platform, with binaries available for Linux and Win32 from Postgresql.org, and Mac OS via Darwin Ports. A PostgreSQL database can, like MySQL, be accessed via APIs written for just about all programming languages, JDBC, and ODBC (which I have quite a bit of experience with.)
• Firebird - this is a newer kid on the block, sort of. It has a very long history, though, since it is based on Borland's InterBase codebase. It's doesn't have nearly the user base, or the amount of available tools, but InterBase is a pretty interesting product, with some good features (like a small footprint, server performance tuning, and a great rollback and recovery system.) It is also cross platform.
• Apache Derby - a DBMS written entirely in Java. This project has a small footprint, and is designed to be easily embedded in other Java projects. It comes with a scripting language and interpreter, called 'ij' which is how you can interact with Derby on the command line. Also, of course, using JDBC is a way to access Derby. I'll be doing a fair bit of experimentation with Derby ('cause I'm curious.)
• SQLite - a small footprint C library that implements an ACID compliant DB engine. It has a command-line tool, and it is possible to use C/C++ and Tcl for database access. Unlike the others, that are released under varied open source licenses, the code for SQLite is public domain.
• There are a few others (see list here,) but they are either research-focused (like Ingres,) developed very little, or have small user bases, and seem not relevant to nonprofit technology.

Nonprofit technology take home lesson: MySQL is certainly the leader - it's most commonly thought of as the 'M' in LAMP (Linux, Apache, MySQL, PHP/Perl/Python), which is a nptech web mainstay. I'd argue that PostgreSQL is a better choice, but for most nptech applications, it doesn't matter - what matters is what your tech/consultant knows, and that's much more likely to be MySQL. The others are most likely of interest to pretty small niche groups, for specific kinds of projects.