Adium

Adium is a free instant messaging application for Mac OS X, released under the GNU GPL and developed by the Adium team. With Adium, you can connect to any number of messaging accounts on any combination of supported messaging services and then chat with other people using those services.

Mac OSX
aMSN

aMSN is a free open source MSN Messenger clone, with features such as:

* Display pictures
* Custom emoticons
* Multi-language support (around 40 languages currently supported)
* Webcam support
* Sign in to more than one account at once
* Full-speed File transfers
* Group support
* Normal, and animated emoticons with sounds
* Chat logs
* Timestamping
* Event alarms
* Conferencing support
* Tabbed chat windows

Linux
Pidgin

Pidgin is an instant messaging program for Windows, Linux, BSD, and other Unixes. You can talk to your friends using AIM, ICQ, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, and Zephyr.

Pidgin can log in to multiple accounts on multiple IM networks simultaneously. This means that you can be chatting with friends on AIM, talking to a friend on Yahoo Messenger, and sitting in an IRC channel all at the same time.

Pidgin supports many features of the various networks, such as file transfer, away messages, and typing notification. It also goes beyond that and provides many unique features. A few popular features are Buddy Pounces, which give the ability to notify you, send a message, play a sound, or run a program when a specific buddy goes away, signs online, or returns from idle; and plugins, consisting of text replacement, a buddy ticker, extended message notification, iconify on away, spell checking, tabbed conversations, and more.

Pidgin runs on a number of platforms, including Windows, Linux, and other UNIX operating systems. Looking for Pidgin for OS X? Try

Linux
Windows