Windows Live Messenger And Yahoo Messenger Are Interoperable
Microsoft and Yahoo have started testing the interoperability between their IM clients: Windows Live Messenger and Yahoo Messenger 8. That means you can add MSN contacts to Yahoo Messenger and the reverse, send messages, view personal status messages, share emoticons, view offline messages.
There are other instant messengers that allow you to login on multiple IM networks simultaneously (for example, Gaim as a desktop client and Meebo from your browser), but this is the first real step in the interoperability of IM networks. The resulting IM network will have 350 million accounts. The next step will be the interoperability between Google Talk and AIM.