The sum total of these Wiki sites about various XML and related topics go some way towards realising Mike Champions idea of an XML Wikipedia. Obviously there's a hell of a lot more to do, but tying together whats around already is a good start.

• CocoDocoWiki -- the Cocoon Documentation Wiki
• RDFWeb Wiki -- a Wiki to accompany the RDFWeb website, IIRC channel, and rdfweb-dev mailing list. Various RDF related topics, including FOAF
• SVGWiki -- Wiki about SVG
• DocBookWiki -- Wiki for DocBook related information
• XML entry on Wikipedia -- the entry for XML in the real Wikipedia. Links to various entries to other XML technologies
• XML related pages on C2 Wiki -- search for XML related pages on the C2 Wiki
• RESTWiki -- for discussions about REST
• Python XML-SIG Wiki -- for collaboratively developing documentation for the PyXML package
• Zope XML -- XML in Zope discussions

Obsolete?

• XML Wiki -- derived from the EFnet #XML IRC channel. Looks to be obsolete.
• BioXMLWiki -- again, looks obsolete

Other

• WikiMLWiki -- for discussion of WikiML, a Wiki parser that created SAX events
• JSPWiki Wiki -- the Wiki site for JSPWiki, the application that runs this site.