Element is a secure collaboration and messaging app based on the Matrix protocol. It’s decentralised to deliver digital sovereignty, and based on an open standard for easy connections and interoperability. It is end-to-end encrypted by default.
With the rise of widespread government monitoring programs, Tox is an easy to use application that allows you to connect with friends and loved ones without anyone else listening in.
Linphone is one of the most famous open source softphones in the world. It is fully SIP-based, for all calling, presence and IM features.
Open source Jabber (XMPP) client with multi-account support, clean and simple interface. Being both free (as in freedom!) and ad-free, Xabber is designed to be the best Jabber client for Android .
The project seems no longer developed since 2019. Last version, 2.6.6, released in October 2019, can be still downloaded from F-Droid, it's no longer available on Google Play Store
ChatSecure is a free and open source messaging app that features OTR encryption over XMPP. You can connect to your existing accounts on Facebook or Google, create new accounts on public XMPP servers(including via Tor), or even connect to your own server for extra security.
Empathy is a messaging program which supports text, voice, video chat, and file transfers over many different protocols. You can tell it about your accounts on all those services and do all your chatting within one application.Discontinued
Empathy is currently unmaintained. Last version, 3.25, released in August 2017, can be still downloaded from the official website.
Technitium Mesh is a secure, anonymous, peer-to-peer (p2p), open source instant messenger designed to provide end-to-end encryption. Primary aim of developing this instant messenger is to provide privacy which is achieved using cryptography and anonymity using Tor network.
WeeChat is a free chat client, fast and light, designed for many operating systems. It is highly customizable and extensible with scripts.
Librem Chat offers you an end-to-end encrypted chat application including text, voice, and video for one-on-one calls and large group chats. Based on the universal Matrix chat protocol ensuring you can communicate with people inside or outside the librem.