Xindice - http://xml.apache.org/xindice

Java based implementation associated with the Apache open source project
No recent development(November 2002)
Documentation incomplete
Implementations with XML-RPC (perl, php and other APIs), Java, Java Servlets, Cocoon
Command-line administrative tools
Requires Java Development Kit - does not work with all versions (Sun 1.4)
Not really discussed in this presentation