Orbeon Forms (www.orbeon.com) is an robust XForms product. They offer both a Community Edition and an Enterprise Edition. The Community Edition lacks a few features that the Enterprise Edition has, but nevertheless it is a great starting point for serious web forms.
Orbeon comes with a Form Runner, which renders an XForms form, collects the data and stores it; and a Forms Designer, which is a browser-based form composer. The interesting thing is that the Forms Designer is in itself an XForms application.
Orbeon is a Java application that runs in the Tomcat webserver context. Orbeon supports a number of underlying databases, such as eXist and MySQL. The Orbeon team uses Stackoverflow to answer users' questions and they do that promptly.