eXist-db is an open source database management system built using XML technology. It stores XML data according to the XML data model and features efficient, index-based XQuery processing.
eXist-db supports many (web) technology standards making it an excellent platform for developing web based applications:
The 1.4 version adds a new full text index based on Apache Lucene, a lightweight URL rewriting and MVC framework as well as support for XProc. Most important, the XQuery engine has seen a major redesign, resulting in improved performance.
The eXist-db has a large community of users and developers, which is very active. The primary medium for asking questions, getting help or posting bug reports or fixes is the project mailing list. To subscribe/unsubscribe visit the mailing lists page on sourceforge.
Previous messages posted on the mailinglists are archived on nabble.com and gmane.org provides a NNTP interface. Additionaly markmail.org provides a nice search interface to the exist-open and exist-commits mailing lists.
The project has an chat box #existdb which is accessible via IRC on irc.freenode.net and via an AJAX based web interface. The channel is open for general discussions on eXist and related subjects (transcript).
eXist is released to the public under the terms of the GNU LGPL license.