Gaim is a multi-protocol instant messaging client for Linux, BSD, MacOS X, and Windows. It is compatible with AIM (Oscar and TOC protocols), ICQ, MSN Messenger, Yahoo, IRC, Jabber, Gadu-Gadu, and Zephyr networks.
Open Source IM Client for Windows. Connects to about all chat networks, including Jabber, AIM/ICQ®, MSN®, Yahoo!®, Gadu-Gadu, Tlen, Netsend and IRC! - Rocks