The XWT Engine is packaged as both an ActiveX control and a Java applet, so you can access XWT applications from all major platforms (Win95/98/ME/NT/2k/XP, Linux, Solaris, MacOS X) without installing any additional software.
I just noticed this link in a Slashdot comment. A cursory over-view of the reference manual shows many good ideas for a small-scale, RAD app builder. Haven't played with it at all, so I don't know how good it is in practice.
Oddly, considering the subject matter, the site does not seem to have a "Click Here to Try It Out!" sample.