An XMPP interface has been exposed for Windows Live Messenger, allowing standards-compliant communication clients to make use of the MSN protocol.
XMPP is the protocol that started life as Jabber, and now powers Google Talk, and is also used by Facebook Chat.
This means that any third-party chat client that can make use of XMPP and supports OAuth2.0 SASL can gain easy access to Messenger's 300 million active users, see which of their friends are online and chat with them in real time.
As a long-suffering third-party chat-client user, I welcome a future without having developers chasing ghosts with Microsoft’s authentication system and various lockouts that occurred over the years.
To Microsoft's credit, the announcement also comes with code samples to help developers on their way.