We ask this question to give you a chance to show you have done more than just glance at our website. We are hoping you will ask questions that show you want know about our business (or the type of work we do), or that show you have experience in our craft and are aware of some of the common challenges we face and how we address them.
Wesley Long's list of questions is great - ask about the things that matter to you (if you were completely satisfied with your current situation, you wouldn't be looking for something else to do, right?)
Please, please, have something to ask. Having nothing at this point in the interview causes us to believe you really don't know anything about us, and that you don't care about knowing anything, either.
The one exception that is acceptable is to respond that all of the questions you brought with you have already been answered during the course of the interview, or beforehand by your primary contact.
Honestly, don't ask about salary or benefits. Many interviewers are not in a position to discuss those. The recruiter/HR rep will be the best person for those topics, and you'll get to discuss them freely with them at the time an offer is made. But do ask about culture, challenges, etc. Do they really like working there? Is there anything in their body language, or inflections of voice when they are talking about company culture?
EDIT: One question to not ask: Is there anything I could have done better in this interview to make you more apt to give me an offer? (True story, the candidate assumed he had failed before he even stood up. He hadn't, until then.)