Marc Laidlaw sez: I have a new favorite obscure puncutation mark:
In 1962, the interrobang (‽), was introduced by the New York publishing establishment as "a twentieth century punctuation mark". The interrobang combined the functions of a question mark and an exclamation point. It received some attention at first, but never caught on, although for a brief period during the 1960s it was added to some typewriter keyboards.