SuperSaaS is a flexible online appointment scheduling system that works with many different businesses and is available in 14 languages.
The basic version for up to 50 appointments, is free, a paid version is available for large users and commercial uses.
The plugin displays a “Book now” button that automatically logs the user into a SuperSaaS schedule using his WordPress user name. It passes the user’s information along, creating or updating the user’s information on SuperSaaS as needed. This saves users from having to log in twice.
Plugin homepage: http://www.supersaas.com/tutorials/wordpress_appointment_scheduling