So far, Genbooks has been the best out there. I have been researching and most of them require clients to log in and I think that might be a turn to some clients. Genbooks does not require clients to log in in order to book appointments. Downside, their free version does sync with outlook or any other type of calendar. In order to have this feature you have to upgrade to the paying plan. If you think your clients will not mind signing in everytime they need to book and appointment, take a lookat these sites:
www.clickbook.com and
www.scheduly.com. These might be a good alternative.
Ivette