XMPP (eXtensible Messaging and Presence Protocol) is a set of open technologies for instant messaging (IM), multi-party chat, voice and video calls, collaboration and content syndication. In short, it is an open, free and decentralized messaging service. Today it is probably best compared to the Matrix messaging protocol, although there are some significant technical differences (XMPP is extensible and lightweight on server resources, whereas Matrix is monolithic and heavy, just to name some of the differences).

XMPP was originally developed in the Jabber open-source community to provide an open, decentralized alternative to the closed instant messaging services at that time. It has a long history, but that does not mean that it's outdated, it is under constant development (extensions being created, updated or closed) and offers modern, secure ways of communication (end-to-end encryption via OMEMO or PGP). It is being used all around the world (WhatsApp messenger is one of the many examples, although WA uses a modified and closed-source version of XMPP not available to the public).

I am a big fan of XMPP for many reasons. If you don't want to use XMPP then I would suggest using Matrix. These are currently the only truly free major messaging services available. Everything else is centralized, evil and/or proprietary.

To get started choose one of the many providers (most of them are free of charge), open an account (you don't even have to enter your e-mail address, it's usually only used for password recovery), download a client or use a web client. On AndroidOS I recommend Monocles Chat or Conversations, where the latter has fewer features, but offers solid quality without many updates throughout the year. For the web browser I recommend Movim. On desktop PCs I recommend Dino, a very good CLI program (console based) for XMPP is Profanity.

• XMPP homepage
• XMPP software overview
• XMPP providers (list still growing)

More links on another page here.