Paolo Redaelli
An open source instant messaging client. Easy to use, reliable, battery friendly. With built-in support for e2e encryption, group chats and media transfer.
Source: Conversations: the very last word in instant messaging
It’s worth noting that it can be obtained in binary form for 0€ on FDroid and that it is free-as-in-freedom software, as you can check out the source code using
git clone https://github.com/siacs/Conversations.git