When we make games like The Line Zen, we use dropbox. We treat it like a library and each person "checks out" the file by emailing/skyping everyone and telling them they are working on it, then letting everyone know when they are finished. We typically have one person that manages the development, so they will appoint one person to work on levels. Having a hierarchy like this can really help avoid confusion.
In the future I'd like to bring a collaborative option as well. Possibly to combine two bbdocs, but its is pretty tough to do. If two people work on very similar changes (say changing button on Main Menu), it's tough to decide which change should be kept. You don't see software like Photoshop, Ableton, Maya, etc typically do this for that very reason.
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