we've used pickatime.com with good response from parents. it's not the most intuitive, but works pretty well for both single and multi-teachers sign ups (we are K-12). Not free, but not terribly expensive either.
Definitely TimeDriver. You have the option to schedule a live meeting, skype meeting, or telephone conference. It works with Outlook or Google Calendar. It allows people to schedule meetings with you automatically and it plugs it into your calendar if the time is available. Just have to remember that if you don't want to be available, block the time out. If not, people will be able to schedule times when you may not want to. There is a small fee after the trial (think it is like $30.00/yr), but well worth it. Has saved me a lot of time chasing sessions and meetings.
Thanks so much! We're currently looking at a few options - none seem to have all the bits we need. We'd like to b able to run an 'error' report identifying those who don't book in. We'll check this out.