Ohm Force has been working on its DAW OhmStudio for quite some time, and now they've made a free beta version available for both Mac and PC. OhmStudio is designed with real-time online collaboration in mind.
"Ohm Studio is the first real-time collaborative digital audio workstation. Start a project, invite musicians and make music together. It's as simple as that.
Download the application and join a worldwide online cohmunity of musicians and producers."