Pidgin

Pidgin (formerly known as Gaim) is a multi-protocol instant messaging client for Windows. It is compatible with AIM, ICQ, MSN Messenger, Yahoo, IRC, 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.