Disa has been discontinued. One of the most popular alternatives is Signal, which is free and open source. Other popular alternatives are Telegram, Element, Pidgin and Tox. There's no shortage of Instant Messengers like Disa, with more than 100 alternatives for Android, iPhone, Windows, Mac and iPad.
FireFloo is an instant messaging communicator combining the open XMPP standard (Jabber, GMail Chat etc.) with the echo protocol for multi-encrypted messaging. Use various methods to send chipertext over open standards like libgcrypt and encrypt your private communication.
Open source anonymous, registration free P2P chat. You communicate directly and no logs are saved on the server. Otr.to is browser based, which means that you don't need to install or ask somebody to install software.
Finch is a console-based IM program that lets you sign on to AIM, Jabber, MSN, IRC, Yahoo!, and other IM networks. It runs on Unixes. It uses GLib and ncurses.
KakaoTalk is a phone number based messenger service that allows users anywhere in the world to chat one-to-one or in a group for free. (no SMS service)
South Korea
End-to-end encrypted chat, VOIP, and video calling used by millions of people.
United States
Phoner provides business owner, sales people, lawyers, doctors, police officers and others with a second phone number to protect their identity and also gives the convenience of multiple phone numbers on one device.
United States
Safe, Fast And Decentralized Messenger.
Zangi Private Messenger is a decentralized messaging app with a focus on security and low data usage.
Austria
EU
BitlBee brings IM (instant messaging) to IRC clients. It's a great solution for people who have an IRC client running all the time and don't want to run an additional MSN/AIM/whatever client.
LimeChat is an IRC client for Windows, Mac OS X and iOS.
Features
Can connect to multiple servers in a window By key operation was rich, achieve a sense of comfortable operation Customization capabilities over the details Stable operation at light Search and comfortable log in.
Japan
qutIM - free open-source multiprotocol ( ICQ, Jabber/GTalk/Ya.Online/LiveJournal.com, Mail.Ru, IRC )
Russia
Coccinella is a free and open-source cross-platform instant messaging client with a built-in whiteboard for improved collaboration with other people.
