I am not sure about the incentive effects of websites such as ratemyprof.com, as Professors with tenure may or may not care about undergrad students. However, they may be more serious about graduate students (supervision, co-authorship etc). Does a ratemyadvisor type website exist? This is important (I think) because relationships between supervisor-supervisee are probably more profound (given the time investment) and have arguably more ramifications on the future (reference letters bear more weight). Any thoughts are welcome!