Social Source Commons

Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.

aMSN is an open source MSN Messenger clone, with features such as: display pictures, custom emoticons, multi language, webcam support, full-speed file transfer, chat logs, conferencing support, event alarms and more.

Instant messaging for Palm OS, can connect to Yahoo, MSN, Jabber, ICQ, from eg. Treo phone

Citadel is a complete and feature-rich open source groupware platform.

Multi-platform and mobile messaging system.

An opensource Voice over IP phone allowing you to do free calls over the Internet.

FWD enables anyone with a broadband connection to communicate over the Internet, whether it be instant messaging, voice, or video.

A suitable GTK+ Jabber client implemented in Python.

Google Talk enables you to call or send instant messages to your friends for free, anytime, anywhere in the world.

iChat is an instant messaging application

Imvite lets you chat with all your contacts from Yahoo Messenger, Windows Live Messenger, AIM and ICQ. It allows you to make free video and worldwide PC to PC calls, read news via RSS and also to watch multiple TV channels at the same time.

Supports video conferencing, voice calls, instant messaging, AVMessaging, filesharing and web co-browsing.

Open source instant messenger service and voip client for Jabber.

Messaging service to multiple devices (email, RSS, text messaging, etc.).

Instant messaging for multiple networks

OTR allows you to have private (not just encrypted) conversations over instant messaging by providing: Encryption, Authentication, Deniability, and Perfect forward secrecy.

Openfire (formerly Wildfire) is a real time collaboration (RTC) server dual-licensed under the Open Source GPL and commercially. It uses the only widely adopted open protocol for instant messaging, XMPP (also called Jabber).

A multi-protocol Instant Messaging (IM) client

Security for electronic communications.

Shinkuro is a small software program that enables teams to securely share files, exchange instant messages, and do simple screen sharing.

Spark is an Open Source, cross-platform IM client optimized for businesses and organizations of all sizes. It includes enterprise-class features such as built-in support for group chat, telephony integration, and strong security.

Instant messaging service.

Instant messenger