A few years ago we started playing around with XForms from the W3C for a web app which required hundreds of custom forms.
As they aren't currently supported natively by the major browsers, what parsers/tools are you using on your projects today?
I'm not really interested in plugins - this needs to be something server side that emulates XForms.
We use XForms for creating user interfaces for SOAP-based web services. Currently we settled with Chiba XForms engine (http://chiba.sourceforge.net/), but Orbeon (http://www.orbeon.com/) actually seems more mature. Both are server-side engines, which convert XForms into HTML on the fly. The validation is performed on server side with the help of AJAX. This puts quite high demands on the server, so I wouldn't bet on those engines when creating sites with heavy traffic. Alternatives are well documented on XForms Wikipedia page: http://en.wikipedia.org/wiki/XForms.