Apps

Comparisons
https://berty.tech/faq
https://simplex.chat/#comparison
https://en.wikipedia.org/wiki/Comparison_of_instant_messaging_protocols
https://en.wikipedia.org/wiki/Comparison_of_cross-platform_instant_messaging_clients
Centralized
Servers are hosted by a provider and cannot be self-hosted. Clients are often provided by same party that hosts the servers and 3rd party clients are not welcome, protocols are not well defined.
Signal - Signal Protocol
Telegram - Custom crypto / off by default!
Discord
Federated
You can self-host your own network and continue exchanging messages with other networks. Clients are often developed independently based on well defined standards.
IRC
irssi
Revolution IRC
XMPP
Conversations
Matrix
Element
e-mail
Delta Chat
Decentralized
Based on some sort of mixing network that hides social network and optionally provides store and forward functionality. Mixnet nodes can be self-hosted.
SimpleX - Mixnet with store & forward
Session - Mixnet with store & forward
Briar - Tor
Distributed
Full peer-to-perr connectivity. IP addresses may leak or be available to friends.
Jami - DHT
Cabal - DAT protocol based
https://tox.chat/ - DHT