Microsoft Cuts Asterisk Ties--What are the Open Source Skype Alternatives?

Microsoft has ended its deal to let the open source Asterisk PBX system work with Skype as of July 26, perhaps due to the launch of its own competing service.

When Microsoft paid $8.5 billion for Skype two weeks ago, they promised to hold it as a separate division and continue supporting non-Microsoft platforms, but users have been skeptical. It didn't take long for the tune to start to change. Maybe somebody should have asked for a pinky swear.

Where should an open source user turn instead? Earlier this week, the GNU Project announced GNU Free Call as an open source alternative, even calling Skype out by name in the project's mission: