FireFox has an XForms plugin that works with the latest nightly builds. Check out http://www.mozilla.org/projects/xforms/ for more info. For IE support, there's an ActiveX control from Novell (http://developer.novell.com/xforms/) and one from x-port.net (http://www.formsplayer.com/).
There's also a JavaScript-based one coming out called FormFaces which looks very promising, especially since there are no plugins required and it works in IE, FF, and Opera: http://www.formfaces.com/
