Pidgin is my main IM tool of choice. It was the best thing on Linux and now I use it on the office Windows box as well. It supports all these networks below. Its been updated to 2.4.0 a few days ago, so if you have it update or try it out.

• AIM
• Bonjour
• Gadu-Gadu
• Google Talk
• Groupwise
• ICQ
• IRC
• MSN
• MySpaceIM
• QQ
• SILC
• SIMPLE
• Sametime
• XMPP
• Yahoo!
• Zephyr