Basically requested about most widely used free relational database I'd be prepared to get MySQL or PostgreSQL or possibly SQLite.
But how about native XML databases? That is most widely used and stable? Have you play one in certain of the projects? Which? Which can you advise for private medium-sized project?
Our organization uses eXist (http://exist-db.org) extensively for the DITA cms.
The merchandise is excellent. Includes full XQuery support, efficient xml indexing along with a good application layer to construct things from.
Per previous comments, the concept Native XML databases really are a fad isn't representational from the market ither. Actually when it comes to NoSQL items MarkLogic server (A local XML database) blows away virtually every other SQL database when it comes to performance and effectively associated with document centric search. Its getting used very extensivly in probably the most performance critical programs of some very large companies. (It is also very costly!
I will not enter in the XML versus JSON factor here an excessive amount of because it is not worthwhile. However, from the storage perspective xml versus json isn't another argument. All databases basically normalize the data lower to very efficinet stores. However xml is an extremely solid standard that's backed with lots of effective technologies.
XPath (parent or ancestor selection anybody?) XQuery XSLT XML Schema XProc XMLDB Namespacing
For those who have a particular requirement for JSON then it's smart to investigate a JSON database. Just remember that XML really is not a fad, and is an extremely solid framework whenever using document centric data.
I believe these were a fad and they are not really popular now. XML will work for data exchange although not exceptional for big data storage.
I have tried personally eXist. I am not declaring that this is actually the best and many popular XML database, however it was Suitable for my reasons. It's easy to install and configure. It's opensource, it supports XQuery, XPath, and (that which was really helpful for my project) it's nice simple Cleaning soap and Relaxation APIs. To be sure using the comment above that XML databases aren't actually advisable for big storages.