Corebird is a modern, easy and fun Twitter client, just what you were looking for, right? (GPL)
Linux
Delta Chat is like Telegram or Whatsapp but without the tracking or central control. Delta Chat doesn’t have their own servers but uses the most massive and diverse open messaging system ever: the existing e-mail server network. (GPL)
Cross-platform
Dino is a modern open-source chat client for the desktop. It focuses on providing a clean and reliable Jabber/XMPP experience while having your privacy in mind. (GPL)
Linux
Empathy consists of a rich set of reusable instant messaging widgets, and a GNOME client using those widgets. (GPL)
Linux - FreeBSD
The goal of Gajim is to provide a full featured and easy to use Jabber client. (GPL)
Linux - FreeBSD - Windows
HexChat is an IRC client based on XChat, but unlike XChat it’s completely free for both Windows and Unix-like systems. (GPL)
Linux - Windows - MAC OS X
With Instantbird, we intend to redefine the way instant messaging is used, so that it works the way you want. Both simple and powerful, it will help you manage all your real time communications. (GPL)
Linux - Windows - MAC OS X
Jitsi (formerly SIP Communicator) is an audio/video and chat communicator that supports protocols such as SIP, XMPP/Jabber, AIM/ICQ, Windows Live, Yahoo! and many other useful features. (Apache)
Cross-platform
Pidgin is a multi-protocol Instant Messaging client that allows you to use all of your IM accounts at once. (GPL)
Linux - FreeBSD - Windows
Spark is an Open Source, cross-platform IM client optimized for businesses and organizations. (GPL)
Linux - Windows - MAC OS X
11 applications