FaceSpan is back
FaceSpan was the first and only application that allowed to build great GUIs for AppleScript back in OS 7/9 days. I used it when it wasn't even called FaceSpan yet (but I can't remember its previous name, now. Update: it was called FrontMost). I don't think I'll use it in OS X, though, because when I need something complex enough to require a graphical interface I'll do it in Cocoa rather than in AppleScript, now. But I'm confident that lots of people will use it, as I'm pretty sure it will be easier to learn than AppleScript Studio.