Back when I was in HR, there was no greater turn-off that the candidate who said "No. I think you've answered all my questions." For me that just showed such a lack of curiosity and I wasn't looking for uncurious people.
HR Guy points to this article on questions to ask during an interview. There are some great questions there and any one of them will (a) impress the interviewer and (b) give you some more information about whether this is a place you actually want to work.
My own personal favorite when I was being interviewed (as opposed to conducting the interview) was this:
"A year from now, how will you evaluate if I have been successful in this position?"
I liked this question because it showed the interviewer that I cared about being successful and it gave me solid information about whether I could be successful. Sometimes the answers were just plain unrealistic - and I'd rather find that out during an interview than months after I took the job.
Remember, looking for a job is like shopping for jeans. You're not looking for the first pair you find - you're looking for the ones that fit just right. When it comes to job search, good questions can help you get there.