Button for Google Calendar™ - See your daily schedule and join meetings in one-click.
AddEvent - AddEvent is an Add to Calendar button for one-time and multiple events.
Add-to-Calendar Button Generator - This open source projects lets everybody create amazing buttons, where users can add events to their calendars.
Microsoft Outlook Calendar - Calendar is a time-management web application by Microsoft.
Vimcal - Vimcal is the world’s fastest calendar, beautifully designed for people who work remotely and live in their calendars.
Super.cc - Quickly add events to your calendar from any email