Via Prawfsblog, I've discovered RateMyProfessors.com. This site is probably no more reliable than student evaluations, but it's interesting nonetheless.

UPDATE: A perceptive reader observed that the study cited in the link for "student evaluations" was itself based on data collected from RateMyProfessors.com. For those interested in independent assessments of student evaluations, I recommend this survey by Michael Huemer of the University of Colorado. See also here and here.

Another reader noted that most of the Volokh conspirators are not rated on RateMyProfessors.com. Hmmmmm.