Vimcal - Vimcal is the world’s fastest calendar, beautifully designed for people who work remotely and live in their calendars.
Add-to-Calendar Button Generator - This open source projects lets everybody create amazing buttons, where users can add events to their calendars.
Clean Google Calendar - Makes Google Calendar look nicer & launches as a desktop app
Add to Calendar Generator - Generate 'Add to Calendar' buttons for your website
Plan - Plan is a better way to organize.
Super.cc - Quickly add events to your calendar from any email