I'm obviously biased (despite not having any equity in the company yet
A free version of NeoCore is available for download from our website. This server has no time limits or other B.S. in its licensing, but does somewhat limit the size of the database (10 GB, which isn't very limited), the number of CPUs used (2), and the number of simultaneous active clients (about 10). There are extra-cost options (obviously; we have to make money somehow) which give you direct support, online backup, larger limits, etc. Support of the free version is through an online forum - free registration is required.
I'm especially proud of our XML query language. It's based on the W3C XQuery language; we've customized it to make it more suited for database access (versus e.g. XSLT addressing), and have left out support for some of the features. Partly we just wanted to keep our query language small (XQuery is a huge language). In one case (schema support) we consider it a feature to leave out a feature, if that makes sense (by not requiring predefinition of XML structures allowed, we can store any XML documents and query them immediately).
Our query language allows retrieval of documents via a path-based language (a la XPath), augmented with a moderate-sized function library, FLWR statements for iterating over result sets, XML constructors for making new documents for output, and built-in sorting by multiple keys.
NeoCore has quite good performance and capacity. Our core technology is based around a set of patented search techniques, and we index absolutely everything (all XML tags and each tag with its data content). Despite this hyperactive indexing, our storage footprint is often smaller than the XML that is stored (chalk this up to the redundancy in XML more than our cleverness).
So, please give it a look if you're interested in XML storage. Please be tolerant of our website, as it seems to be a no-no in product marketting to mention what the product actually does (I'm reminded of Kermit the Frog in "The Muppets Take Manhattan"). You might get something very useful for no cost, other than your time.