Pulse 24/7 - Booking & scheduling mobile app for service providers
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Time Slot Booking Calendar PHP - Time Slots Booking Calendar PHP for online appointment booking and class scheduling.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Appointment Booking Calendar - With the Appointment Booking Calendar you can accept online bookings from a set of available time-slots in the calendar.
Bigtent - BigTent is a web-based organization and communication portal for groups.