Remind1 - Save nerves, time and money - send automated appointment reminder messages from the calendar you already use. Google Calendar & Outlook / Office 365 Integration.
FullCalendar - FullCalendar is an open-source JavaScript event calendar.
Remindy.me - Never forget anything!
Dudle - Dudle is an online scheduling application, which is free and open source.
Appointment Reminder - Automated SMS/Voice and Email Reminders.
Mobile-Calendar - Reservation management system. If you rent accommodation, the mobile-calendar app was made for you!