We are currently working on a website for a Chiropractor and Podiatrist and one of the requirements was a web based calendar that could be integrated with the website.  Now there’s many out there but after reviewing many options, we have gone for Appointy.com.  Their initial service is free, but it can be upgraded to include more and more features.  Currently the client doesn’t want the public to be able to book appointments online, but the facility is there and it’s better to start with a product and grow into it, than having to change to a new system later down the line.  If enabled, visitors can register with an email address or with a Facebook account.  Messages such as reminders are sent automatically via email for you and there’s even a text message option (paid).  The calendar can be easily amended from any computer and there’s also a mobile version of the site.  If your after something similar, it’s worth registering for a free account.