On my laptop I have most (not all, because something is always missing) software I need for my day job and some add-on tasks, including a database system. Right now it is DB2 9.7 Express-C, a free database system where I very often use the included pureXML feature to test out XPath or XQuery expressions or quickly manipulate XML data. The latter can be done by importing the XML files, applying some XQuery magic, maybe even combine it with relational data and then being done. Other people use it to find the closest ATM - always good to have DB2 pureXML handy.
I also took a look at other free database systems. First, there was Oracle XE. While it seems to offer XML support, the software is based on an old release level (10g R2). Support is through a forum, but requires registration to even look inside to see how the support is. Nothing for me.
Next on my list was MySQL which I use in some LAMP environments and - being open source - has lots of community support. However, MySQL's XML support is limited in that sense that, e.g., XQuery expressions are not supported and that XPath evaluation is embedded into SQL in a non-standard way. So nothing for me again.
Last on my list during my evaluation was PostgreSQL. Here the picture is basically the same as for MySQL. PostgreSQL's XML support is limited again in terms of functionality and how, e.g., XPath expressions are embedded into SQL.
DB2 Express-C is free, is based on the latest DB2 release, has an open support forum (no tricks), and offers the broad XML support that the pureXML feature has. So it is DB2 Express-C on my laptop.
[Don't get me wrong when I talk about PostgreSQL and MySQL. I especially like that they added XML support over the years because it widens the common functionality available across products and leads to more XML-related skills among developers and DBAs.]