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"Google Talk enables you to call or send instant messages to your friends for free--anytime, anywhere in the world." (from the talk.google.com page)
Windows Live Messenger is an instant messaging program from Microsoft. Formerly MSN Messenger Service.
Native alternatives for non-windows platforms
- Gaim (works with Yahoo!, MSN, ICQ and many others)
- Kopete (supports advanced MSN features. Part of KDE's PIM package (kdepim))
- aMSN (supports webcam, nudge, inkdraw and other advanced MSN features)