The interrobang is basically a combination of a question mark and exclamation mark and question mark as one symbol; instead of "!?", it is "‽", or "!?!?!?" it is "‽‽‽".
Many people have adopted the "/s" to indicate sarcasm as a shorthand, I think there was a symbol created for that but it hasn't caught on as much.
What other alternative punctuation do you think would be useful?