Pidgin

Pidgin is a universal chat client that supports a massive amount of protocols;

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

However it also has a good number of third party plugins providing things like facebook and twitter integration. It is a perfect replacement for the many messaging clients some people use at once. I even find it preferable to most, I certainly find it less annoying than windows live messenger (you won't find any ads. on Pidgin) so even for users of a single protocol it may be beneficial.

Pidgin can keep track of multiple accounts and allows you to configure them to either automatically or manually login. It easily becomes a one stop shop for all your online communication needs.