New features available in this version include:

Add-ins for Windows Messenger
This new architecture allows a variety of third party developers to extend Windows Messenger version 4.6 with exciting features, such as e-mail integration, chat, mobile text messaging, and more. These add-ins can make Windows Messenger even more useful and fun for you, while the add-in architecture provides new opportunity for others in the industry to create new, innovative offerings.

Additional computer-to-phone service options
Enterprise organizations can customize the phone-dialer window in Windows Messenger to route computer-to-phone calls through the service provider your enterprise IT department selects. This can reduce the cost of long distance voice calls for your organization and enable new services that integrate with Windows Messenger. -And you benefit from the convenience of calling right from your computer.

Other improvements
This update also provides some enhancements to Help, clarifications within error messages, more flexibility in re-sizing the Conversation Window, and other minor performance improvements.