Kontact - Kontact is the integrated Personal Information Manager of KDE.
Truecaller - Find a person by a name or phone number worldwide for free using Truecaller.
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.
Kontact - KAddressBook - KAddressBook stores all the personal details of your family, friends and other contacts.
FullContact Card Reader - Automagically scan biz cards into LinkedIn/Gmail contacts
Nextcloud Contacts - The Nextcloud contacts app is a user interface for Nextcloud's CardDAV server.