We also often would like to know what is good qualiity. A good measure of that is what is popular. For instance, the New York Times Best Seller List is a good place to go to find a good book to read. While popularity isn't a perfect measure of quality (for how does something new make it onto the list if it isn't already popular?), it is a pretty good filter for what isn't good.
This is called SocialNavigation (Dourish and Chalmers, 1994). An excellent book on this subject is Munro, Höök, and Benyon (1999).
Wexelblat (1998) provides a reasonable framework for looking at SocialNavigation online.
Dourish, P. and Chalmers, M. (1994) Running out of space: Models of information navigation. In HCI '94, Glasgow.
Munro, A. J., Höök, K., and Benyon, D. (eds.) (1999) Social navigation of information space. London: Springer-Verlag.
Wexelblat, A. (1998) History-rich tools for social navigation. Available from http://web.media.mit.edu/~wex/Footprints2/fp-v2.html