Very impressive, but I am not sure his comment about Mathematica is right. The Wolfram Demonstrations project is about aiding such explorations in Mathematica. It may not look as pretty, but it's definitely capable of enabling explorations using the Manipulate[] command.
Work is very nice by web standards, but Mathematica's version of interactive documents is much more sophisticated. Strange that he seems to be unaware of what Mathematica can do, given the rant on tools at the end. Its pretty much exactly what he wants, only just proprietary and not on the web.
"Its pretty much exactly what he wants, only just proprietary and not on the web."
Which is exactly what he doesn't want. The authoring tools should be accessible to everyone.
Secondly, after exploring his site I'm guessing Bret would reject Mathematica even if it were free & open source. Because even then it's not accessible to everyone - you still need to know how to program.
All I'm saying is that its a shame he doesn't seem to know about what Mathematica has done in version 6 and beyond, since its seems squarely up his alley... which is ironic because he specifically calls out Mathematica in the article for not being interactive enough.
http://demonstrations.wolfram.com/
Example: http://demonstrations.wolfram.com/DigitalTiltShiftPhotograph...
http://reference.wolfram.com/mathematica/ref/Manipulate.html