We love remembering how Art's dad used to say, "You shuah (sure)?" as an exclamation of surprise. It made us smile every time he'd say it.
In California we noticed that Art's sister and brother-in-law often say, "Is that right?" It all came together when we had guests from San Francisco and they said, "Is that right?" also.
In Illinois I think we say, "Really?" or "Are you serious?" or "Oh yeah? or "No Kidding?" or "No way..." (That's my son-in-law).
I've also heard "You don't say" somewhere but I don't remember where.
In Hawaii I've heard, "No joke?" or "Fo real?"
I just find this so fascinating. What do you say in your part of the country?