The beta of Windows Live Messenger, which is the next generation of MSN Messenger, was previously available for beta testing by invitation only.
Martin Taylor, corporate vice president of Windows Live and MSN at Microsoft, said: "Windows Live Messenger is a key part of the foundation of Windows Live.
"The several million consumers already in the managed beta have been giving us great feedback, and we're excited to make the beta of Windows Live Messenger to users worldwide."
The public beta release of Windows Live Messenger has a revamped look and feel including easy personalisation features and a toolbar that provides one-click access to favourite services.
The beta application integrates Windows Live Call with Verizon's Web Calling service to allow users in the US, UK, Germany, France and Spain to link directly to the Verizon VoIP web telephony service.
As of next week, this calling service will be extended to Austria, Belgium, Finland, Ireland, Italy and The Netherlands.
Consumers can use their cordless phones from anywhere in the home, Microsoft said, even while away from the PC.
The application also features unified contacts so that customers can see and search all their contacts with the unified contact list accessible through Windows Live Messenger.
Improved support is also provided for video conversations via a free synchronised audio and video service.
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