Michael Kay's Saxon 7.8: http://saxon.sourceforge.net/
XQuisitor: Keep an eye on Cafe con Leche
Bell Labs' Galax: http://db.bell-labs.com/galax/
Xavier Franc's Qizx/open: http://www.xfra.net/qizxopen/, an open source impementation of XQuery, written in Java. It conforms to XQuery Basic with Static Type Checking.
Software AG's QuiP: http://www.softwareag.com/developer/quip/
Quilt: http://www.almaden.ibm.com/cs/people/chamberlin/quilt.html
Kweelt: http://kweelt.sourceforge.net/
Software AG's Tamino: http://www.softwareag.com/tamino/
Ipedo: http://www.ipedo.com/
Cognetic Systems's XQuantum: http://www.cogneticsystems.com/xquery/xquery.html
Fatdog's XQEngine: http://www.fatdog.com/
GAEL's Derby: http://www.gael.fr/derby/
Qexo (Kawa-Query): http://www.qexo.org/ Compiles XQuery on-the-fly to Java bytecodes. Based on and part of the Kawa framework. Open-source.
IPSI's IPSI-XQ: http://ipsi.fhg.de/oasys/projects/ipsi-xq/index_e.html
Microsoft's XML Query Language Demo: http://xqueryservices.com
Nimble Technology's Nimble Integration Suite: http://www.nimble.com/
OpenLink Software's Virtuoso Universal Server: http://demo.openlinksw.com:8890/xqdemo
Oracle's XML DB: http://otn.oracle.com/sample_code/tech/xml/xmldb/xmldb_xquerydownload.html
QuiLogic's SQL/XML-IMDB: http://www.quilogic.cc/xml.htm
Chris Wilper's XQuench: http://xquench.sourceforge.net/ Open-source.
X-Hive's XQuery demo: http://www.x-hive.com/xquery
XML Global's GoXML DB: http://www.xmlglobal.com/prod/xmlworkbench/