Previously called MSN
Messenger, Microsoft’s Windows Live Messenger (WLM) is an advertisement-supported instant messaging service. It currently has more than 27 million active registered users worldwide. WLM offers file sharing, PC-to-PC calling, time-stamped messages, offline notifications and other features. In 2006, Microsoft and Yahoo announced an interoperability agreement which would allow users on either system to communicate with each other. WLM is available for the PC and mobile applications also.
See also “Public IM Services” in the Quick Reference section of Webopedia.