WhatsApp is an instant messaging service that allows users to send text and voice messages, make voice and video calls, as well as share various forms of media like images, videos, geolocations, documents, etc.
Viber is a free instant messaging app with cross-platform capabilities used for quick communication with family, friends, colleagues, etc. All one-on-one chats, group chats, and calls on Viber are end-to-end encrypted.
Messenger is an app that allows users to keep in touch with each other using messages with links and media files, as well as search for other users and groups. Developed by Facebook, Messenger has over a billion monthly active users.
Signal is a secure, privacy-focused messaging app that supports text messaging and voice/video calls, either one-on-one or in groups. All conversations on Signal are end-to-end encrypted. All you need to join is a phone number.