Things - Things is an easy to use task manager.
Button for Google Calendar™ - See your daily schedule and join meetings in one-click.
Motion Firefox - All-in-one time management tool in Firefox
Calndr.link - Dead simple & free add to calendar links + API
Taskable - Using multiple work apps leads to tons of context switching, breaking flow and killing productivity.
Add-to-Calendar Button Generator - This open source projects lets everybody create amazing buttons, where users can add events to their calendars.