But it's definitely a niche product, which we've seen isn't something Apple is interested in. Supercard.us is down right now, so I'm not sure whether it's on the market or not. As Apple has discovered, the vast majority of computer users aren't interested in creating software; they're content consumers. SuperCard probably got as much use for making quick mockups before building a "real" application as it did by amateur developers.
One thing that I'll give HyperCard is that it made it easy for me to play around with programming while I was in elementary school. Today's programming tools are largely not that accessible.