XForms
XForms looks cool and everything, but I've found only one processor so far, and that runs as an IE6 plug-in. I'm not sure that I would want to prescribe IE6 and a plug-in to the users just to fill in forms. Maybe some kind of do-it-all-on-the-server approach might be better, but then there wouldn't be any XML being posted from the client, which kind of defeats the object. I've got some more thinking to do...