Pidgin is an easy to use, multi-protocol chat client client used by millions. Connect to AIM, MSN, Yahoo, XMPP, and more chat networks all at once. It has built-in support for NSS, offering client-to-server message encryption for protocols that support it.
The goal of Gajim is to provide a full featured and easy to use XMPP client. Gajim works nicely with GNOME, but does not require it to run. It is released under the GNU General Public License.
Empathy is a messaging program which supports text, voice, video chat, and file transfers over many different protocols. You can tell it about your accounts on all those services and do all your chatting within one application.Discontinued
Empathy is currently unmaintained. Last version, 3.25, released in August 2017, can be still downloaded from the official website.
Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Google Talk, AIM, Jabber, ICQ, Facebook, MySpace and Twitter on ALL popular mobile platforms.
The widest selection of unique features that you won't find in any other IM client!.
Messenger for Desktop is a wrapper for the official client messenger.com. Therefore it works like a regular browser which can only navigate to the messenger.com web app. MFD doesn't touch your messages, account or personal data. All that is handled securely by Facebook.
Be careful during the installation: https://alternativeto.net/discussions/applications/11718/be-careful-in-the-installation/
MCabber is a small XMPP (Jabber) console client. It includes features such as SASL/SSL/TLS support, MUC (Multi-User Chat) support, history logging, command completion, OpenPGP encryption, OTR (Off-the-Record Messaging) support, dynamic modules and external action triggers.
Coccinella is a free and open-source cross-platform instant messaging client with a built-in whiteboard for improved collaboration with other people.Discontinued
The last version 0.96.20 is from late 2010, but the software can still be downloaded from their website.
BeagleIM is a lightweight and powerful XMPP client developed by Tigase, Inc. It provides an easy way to start using the XMPP Protocol (formerly known as Jabber) if you've never used it before. Veterans of the protocol will find many features here they are familiar.
Ayttm is an instant messaging (aka chat) client that provides all-in-one chat functionality for several major instant messaging services from one simple program. Ayttm "Contacts" allow you to refer to several accounts of the same person from a single contact name...