Google Contacts - Google Contacts is Google's contact management tool that is available in its free email service Gmail, as a standalone service, and as a part of Google's business-oriented suite of web apps Google Apps.
Google Calendar - Spend less time managing your day & more time enjoying it
Nextcloud Contacts - The Nextcloud contacts app is a user interface for Nextcloud's CardDAV server.
Apple Contacts App - Access and edit your contacts from personal, business, and other accounts with the Contacts app.
Microsoft People - People in Windows 10 puts all the ways you connect with all your friends, family, colleagues, and acquaintances in one place, so it’s faster than ever to keep in touch.
Lightning Calendar - Organize your life — it’s about time!