Vimcal - Vimcal is the world’s fastest calendar, beautifully designed for people who work remotely and live in their calendars.
SavvyCal - A scheduling tool both the sender and the recipient will love.
Plan - Plan is a better way to organize.
Calendly - Say goodbye to phone and email tag for finding the perfect meeting time with Calendly. It's 100% free, super easy to use and you'll love our customer service.
Add-to-Calendar Button Generator - This open source projects lets everybody create amazing buttons, where users can add events to their calendars.
FreeBusy - Easiest way to share availability across calendars for free