SuperSaaS is a flexible appointment scheduling system that works with many different businesses. The basic version is free.
This is the most comprehensive booking system I could find for a website where appointments scheduling was needed. It also gives payment options and it is possible to send reminders by e-mail or SMS. Highly recommended.
Great plugin, great online booking system. Saved me a lot of time by being able to refer customers to my website to make a booking instead of handling bookings by phone.