The best opensource xml database I can find at the moment Exist. Written by a mastermind and good enough to use in almost every project regarding xml. Supports xinclude and limited xquery. Works so well with cocoon its untrue and has built in support for webdav. Amazing stuff, cant wait to find more time to experiement with it.
Querying exist is easy as pie.
supports cocoon views – very useful. as in http://127.0.0.1:81/exist/xmldb/courselets/?xpath=document(*)//q&cocoon-view=content (thank you miles)