eXist is a native XML database featuring efficient, index-based XPath query processing, extensions for keyword search, XUpdate support, and tight integration with existing XML development tools. The database is lightweight and may be easily deployed in a number of ways, running either as a stand-alone server process, inside a servlet engine, or directly embedded into an application.
BaseX is a light-weight, high-performance, and scalable XML database system and XPath/XQuery processor, including full support for the W3C Update and Full Text extensions. An interactive and user-friendly GUI frontend gives you great insight into large XML data instances. It is platform independent and works out of the box.
Zorba is a general-purpose XQuery processor implementing in C++ the W3C family of specifications. It is not an XML database. The query processor has been designed to be embeddable in a variety of environments, such as other programming languages extended with XML processing capabilities, browsers, database servers, XML message dispatchers, or smartphones. Its architecture employes a modular design, which allows customizing the Zorba query processor to the environment’s needs. In particular, the architecture of the query processor allows a pluggable XML store (e.g. main memory, DOM stores, persistent disk-based large stores, and S3 stores).