Pidgin (formerly Gaim) 2.5.8

Gaim is a multi-protocol instant messaging client compatible with AIM (Oscar and TOC protocols), Yahoo, ICQ, MSN Messenger, Gadu-Gadu, IRC, Jabber and Zephyr networks.
Gaim 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.
Gaim supports many features of the various networks, such as file transfer (coming soon), away messages, typing notification, and MSN window closing 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, and more.