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.
I do not use them and as they are not supported by any major browsers I doubt that anybody else will use them very often either.
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 a fly. The validation is performed at server side with the help of AJAX. This puts quite high demands on 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.
As far as I've understood, XForms is a natural fit to the current flavour of REST-based architectures while addressing most of the major issues with complex form development in a pretty neat way.
It's sad that people have largely forgotten about it :(
That said, there are Javascript-based xforms engines like Ubiquity that would help in getting cross-browser xforms support. And the recent development of high-performance Javascript VM's would give such engines great performance as well.
It is also possible to convert XForms to XHTML+Javascript with just an XSLT transformation so it can be done on client-side without plug-in. Have a look at http://www.agencexml.com/xsltforms/. It's an opensource project : http://sourceforge.net/projects/xsltforms