Today Microsoft launched the Windows Live Messenger, one of 20 new Services in the Live concept. Windows Live Messenger goes beyond the traditional instant messaging (IM) service, enabling people to connect and share, with free PC-to-PC calls and inexpensive calls from a PC to phones around the world, video calling, easy sharing with Sharing Folders, and more.
As of now Microsoft invested one billion into their new Live Network and they are planning further improvements. The whole platform is pierced with the hyped AJAX Technology.
You can download it here: Windows Live Messenger