XForms, a combination of two of the most successful experiments ever performed with the Web: XML and forms.
For my current contract they could have saved months of effort by using XForms instead of standard HTML forms. We're talking thousands and thousands of lines of JSP code that could be thrown in the trash. I love nothing more than throwing code away ... love it.
It's not surprising that XForms is raising hackles and ruffling feathers since it has the potential to displace a language that a lot of people have spent a lot of time learning and building businesses around. But just as I gave up assembler programming to use C, so I now gladly drop spaghetti-script for XForms. It's not perfect and there are plenty of new features that I would like to see introduced, but its most important role has been to demonstrate that we don't need to be wedded to the old approaches for ever, and that new solutions are possible, and here today.