After much struggling with heirarchical menus in DHTML and dealing with numerous bugs, we've been searching for an alternate solution. We tried embedding a browser in a wxPython or wxPerl app, but no dice. Finally, a coworker asked me about the Mozilla development tools. I had heard of XUL (rhymes with 'zool'), but never tried it. After a bit of research and playing around with it over the weekend (there's a great tutorial on XulPlanet), I came in this morning and tried to implement it. I added the XUL mime type to my Apache httpd.conf file, restarted Apache, and changed my frameset menu to point to my new XUL document. It works perfectly, is easy to use and gets us the cross-platform support that we lacked (but we can't use IE, aw shucks).
I'm also concerned about shipping a production application with XUL, but it's been out for a couple of years now and I'm crossing my fingers about stability. Everything else has been a failure, though, so it looks like this is a good way to go