Skype for Web
blogs.skype.com
blogs.skype.com
"look out for access to Skype for Web on Skype.com in the coming weeks"
and
"We’re making Skype for Web available to small number of existing and new users to begin with, and gradually rolling out worldwide in the coming months – look out for an invite when you sign in to your Skype account on Skype.com."
Its tricky - you need a web portal for audio and video streaming, and some special servers to bridge to the common media servers for switching. Most open source is about playing streams, so there are issue with latency and dynamic configuration (bugs in the Open Source libraries).
Edit: the description says "Mac OS: Safari 6 and above." Safari doesn't support WebRTC and Skype says "no downloads". So either this document is incorrect or Skype is using Flash.