My pal Jason Moore discusses using paper prototypes as a fast way to get usability feedback. I found it interesting that by going low-tech, you actually get better feedback, because people are more willing to criticize a paper model than running code. (And another advantage of the paper model is that you can make changes on the fly. If during the session you get the idea, "Maybe if I did it this way," you can grab a piece of paper, write on it, and insert it into the session instantly. Try doing that with running code.)