Microsoft has released a stand-alone Windows Live Messenger client for Windows Phone.
The app is localized for China and nowhere else, but is available to download world-wide.
The app allows users to use various accounts, is optimised for different network types and low power usage.
At this stage it is not clear if the app will be more widely localized, or is merely a response to social network integration issues in China.
If you can read Chinese however the app can be found in Marketplace here.