It looks like Apple finally retired Hypercard. I was never a huge fan of Hypercard, perhaps this is because I experienced Squeak and MS-Visual Basic (version 3.0) before I had used Hypercard and was tainted by the programmers environment. I had also used Delphi which was one of my favourite tools, i.e., it was extremely similar to VB but with ObjectPascal (this information is probably way out of date and inaccurate since I haven’t touched Delphi in over 8+ years).
Kevin Altis provides links to Runtime Revolution and PythonCard as Hypercard replacements. Rick Ford provides a comprehensive discussion of Hypercard alternatives from readers at Macintouch I have been using Macromedia’s Flash, Real Basic and Apple’s Interface Builder to do UI prototyping. Real Basic is the tool that I prefer these days, and it works cross-platform now with RealBasic 5.5.