Cronofy - Unified Calendar API for developers
Fantastical 2 - Fantastical, the Mac calendar app you'll enjoy using. Quickly create new events with natural language input and more.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Google Calendar - Spend less time managing your day & more time enjoying it
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Superpowered - Power through the day without your calendar