Pidgin

Pidgin (formerly known as Gaim) is a multi-protocol instant messaging client for BSD and Windows. It is compatible with AIM (Oscar and TOC protocols), ICQ, MSN Messenger, Yahoo, IRC, and Jabber, Gadu-Gadu, and Zephyr networks. Pidgin users can log in to multiple accounts on multiple IM networks simultaneously. This means that you can be chatting with friends on AOL Instant Messenger, talking to a friend on Yahoo Messenger, and sitting in an IRC channel all at the same time. 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 plug-in, consisting of text replacement, a buddy ticker, extended message notification, and iconify on away.