HyperTalk

on OpenStack
  show message box
  put "Hello World!" into message box
end OpenStack

HyperTalk again...but better!

I'm the author of HyperTalk and I'd like to improve your hello world example.  
The example you have now is:

on OpenStack
  show message box
  put "Hello World!" into message box
end OpenStack

But since the message box is shown when needed automatically, and is also the 
default destination of a put command, you could get exactly the same result from 
this shorter script:

on openStack
  put "Hello World!"
end openStack

Also, since commands can be typed directly into HyperCard's message box, you 
could really just type:

put "Hello World!"

without making an openStack handler at all.

Also, if you wanted to have the message come up in a new dialog box you could do:

answer "Hello World!"

Finally, if you wanted it to repeat forever as some of the examples from other 
languages do, you could use a repeat statement.  The message box won't take multiple 
line statements so this would require making a handler, but you wouldn't normally use 
openStack.  I think a better choice would be to make up a new message, say "hello":

on hello
  repeat forever
    answer "Hello World!"
  end repeat
end hello

This little handler will begin running when you enter the message "hello" and will 
repeated put up a dialog box that says hello world and let's you click OK.  It will not 
stop until you hit command-. on the Mac.

--Dan Winkler