A polite formula used when one has not heard or understood what has been said, or to indicate surprise.
Used as a polite farewell to a visitor, inviting a return visit.
Used either to belittle the issuer of a statement/question, or sarcastically to indicate utter agreement, and that the statement being responded to is an extreme understatement. The intonation is changed to distinguish between the two meanings - implied dullness for belittlement, and feigned surprise for utter agreement.
Used to express doubt or confusion.
Used to express amusement or subtle surprise.
Used to create a tag question.
Used to indicate that one did not hear what was said.
Used to reinforce a question.