Kontact - Kontact is the integrated Personal Information Manager of KDE.
PowerSchool - PowerSchool provides a K-12 education technology platform for operations, classroom, student growth, and family engagement.
Nextcloud Contacts - The Nextcloud contacts app is a user interface for Nextcloud's CardDAV server.
Clever - syncing between education applications for K-12 schools
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.
Claroline - Claroline is a collaborative eLearning and eWorking platform.