Michael Kay's Saxon 8: http://saxon.sourceforge.net/
XQuisitor: http://www.cafeconleche.org/xquisitor/
eXist
Berkeley dbXML
Bell Labs' Galax: http://www.galaxquery.org/
Xavier Franc's Qizx/open: http://www.axyana.com/qizxopen, an open source impementation of XQuery, written in Java. It conforms to XQuery Basic with Static Type Checking.
Mark Logic Server: http://www.cerisent.com/products/ml_server.html
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://xqengine.sourceforge.net/
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
Oracle Database 10g Release 2: http://www.oracle.com/technology/tech/xml/xquery/index.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