The World-Wide Web Consortium (W3C) http://www.w3.org/ maintains the standards

Specifically, http://www.w3.org/MarkUp/Forms/ is the XForms subsection of the W3C site.

XForms Institute http://xformsinstitute.com/ is an online tutorial to XForms. This site also has an interactive XForms validation tool at http://xformsinstitute.com/validator/

The IBM® Workplace Forms™ product family consists of a server, designer and client viewer that together enable the creation, deployment and streamlining of XML forms-based processes.

The CoverPages http://xml.coverpages.org/ are a public access reference collection supporting the SGML/XML family of (meta) markup language standards and their applications. The principal objective in this knowledgebase is to promote and enable the use of open, interoperable standards-based solutions that protect digital information and enhance the integrity of communication.