Even if something functions without errors, if it doesn't make sense to its users then it is as bad as broken.

Tips on Moderating Listening Labs from goodexperience

http://usability.gov/methods/usability_testing.html

http://www.useit.com/papers/international_usetest.html

http://www.cu.edu/~irm/stds/usability/

http://anarch.ie.utoronto.ca/