FLOSS Messengers

Generic

https://whispersystems.org/blog/the-new-signal/

https://matrix.org/

https://github.com/67P/hyperchannel

https://www.waartaa.com/

http://demo.shout-irc.com/

http://telegram.org/

http://whatsapp-desktop.com/#download

https://www.jsxc.org/

https://vector.im/

https://chatsecure.org/

https://github.com/surespot

node / electron

https://github.com/moose-team/friends

https://github.com/haadcode/orbit

https://wire.com/en/

skype like

https://github.com/irungentoo/toxcore

https://github.com/BelledonneCommunications

Slack like

https://www.zulip.org/

https://github.com/RocketChat/Rocket.Chat

http://www.mattermost.org/

https://sdelements.github.io/lets-chat/

facebook

https://github.com/sindresorhus/caprine

chat buttons

https://github.com/gitterHQ/sidecar

IRC based

http://ircanywhere.com/

https://github.com/thelounge/lounge

https://github.com/quassel/quassel

https://smuxi.im/contribute/

Encryption

https://github.com/keybase/client

Widgets

https://github.com/idoco/intergram

Linked Data

https://github.com/linkeddata/solid-app-set/blob/master/chat/chatPane.js