Signal is a popular private messaging, voice call and video call service. All communication between its users is end-to-end-encrypted by default.
Note: You register with your phone number. That’s why Signal requires you to activate your account via a smartphone app.
XMPP (also known as Jabber) is a widely-used free and open internet messaging standard. It’s very easy to use: just register an XMPP account and install an XMPP app. No phone number required.
Because it’s federated, there are lots of sites you can register on. It doesn’t matter if your friends sign up on a different site, all the sites talk to each other.
If these apps and sites are too complicated, try the much easier Zom and Quicksy alternatives. They’re part of the XMPP network as well.
Quicksy is an easy-to-use chat app. You can register with just your phone number.
It’s based on the XMPP standard, so if you know someone with an XMPP account you can communicate with them as well.
Zom is an extremely easy to use and free open source messenger app. You don’t need to register to use it, just think of a username.
It’s compatible with the popular XMPP standard used by many other messenger apps, so if you know someone who uses one of those other apps you can message them with Zom.
Element / Matrix
Element (previously Riot.im) is a libre instant messaging client based on the Matrix protocol. It includes text chats, audio/video calls and file transfers.
The Matrix protocol has a federated design and allows bridges to other communication apps. Furthermore, anyone can set up their own Matrix server and use it to collaborate with other people’s Matrix servers.
Jami (formerly “Ring”) is a libre easy-to-use alternative to Skype, with voice call, video call and messaging.
It is open source and designed to respect its users’ privacy, with calls going directly from one user to another without using servers. It gives all its members a unique code, which they can share with others, so that they can add each other on the service.