I haven't looked exhaustively, but there are solutions out there to book appointments with your doctor or dentist, and there must exist some solution to book an appt with your professor, but what's needed here is something simpler, i think. it would be interesting to see what others say.
Yeh I get what you mean; something simple where you can say "I want a 4 hour block here - divided into 30 minute segments - so that people can type in their details and select a block and brief out what they want to discuss".
We had a similar system at uni for booking tutor time - but as you say it was overly complicated.
See also http://officehours.firstround.com/ and I think Fred Wilson does office hours now too. I wonder what tool, if any, PG uses?