If I asked about most popular free relational database I'd expect to get MySQL or PostgreSQL or perhaps SQLite.
But what about native XML databases? Which is most popular and stable? Did you use one in some of your projects? Which one? Which one would you advise for personal medium sized project?
I think they were a fad and none are really popular now. XML is good for data exchange but not that great for large data storage.
I have used eXist. I'm not claiming that this is the best and most popular XML database, but it was OK for my purposes. It's simple to install and configure. It's opensource, it supports XQuery, XPath, and (what was really useful for my project) it has nice and simple SOAP and REST APIs. I agree with the comment above that XML databases are not really a good idea for large storages.