AddEvent - AddEvent is an Add to Calendar button for one-time and multiple events.
Calndr.link - Dead simple & free add to calendar links + API
Microsoft Outlook Calendar - Calendar is a time-management web application by Microsoft.
Button for Google Calendar™ - See your daily schedule and join meetings in one-click.
Digital inspiration - Digital Inspiration is a popular how-to website that covers computer software, mobile apps, video...
Add to Calendar Link Generator - Create buttons for Google Calendar, Office 365 and Apple Cal