Dudle - Dudle is an online scheduling application, which is free and open source.
Toggl - Toggl is an online time tracking tool. It features 1-click time tracking and helps you see where your time goes. Free and paid versions are available.
Pulse 24/7 - Booking & scheduling mobile app for service providers
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Time Slot Booking Calendar PHP - Time Slots Booking Calendar PHP for online appointment booking and class scheduling.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.