Calendly
An easy to use online booking system that you can integrate with your own calendars. Simply post the code into your website or emails and clients can access your preset schedule from anywhere.  
There is a free version to begin but it is very limited. 
https://calendly.com​

Practice Better
Practice Better is the complete nutrition practice management software for wellness professionals.
Everything from scheduling, invoicing, intake forms, client accountability and info sharing.
Free to start and set up.  As you build your client list, you can move to the paid version.
Recieve your 25% discount on any annual membership.  Thats up to a $200 savings every yea​r!
USE CODE: CAINPRO25
https://practicebetter.io​

ScheduleOnce
Online scheduler.  It’s easy to set up and it syncs with your google Calendar. It’s a quick easy way to set up sessions with people and send them reminders without the back and forth of email.
http://www.scheduleonce.com/

​Mindbody
From appointment booking, tracking, client information and marketing to point of sale services, and mobile apps to help you run your business smoothly while maximizing
retention and revenue.   
https://www.mindbodyonline.com

Acuity Scheduling
An online scheduling tool that allows you to take payments and intake forms as well. Integrates with the major email marketing and payment services.
https://acuityscheduling.com​