I knew there was another product I'd meant to mention, but had forgotten what it was until I went through the stack of business cards and brochures I'd collected. That one is a musical-collaboration service called
SkyTracks.io.
I have had no interest in online collaboration tools, since I'm perfectly happy with DropBox and email. But these folks were on the way to my next appointment and they looked bored, so I stopped to chat with them. To my surprise, I came away thinking it might just be something I'd actually be interested in.
The problem with traditional collaboration methods is that one person has to "own" the project and manage the addition of files submitted by remote contributors. Those contributors typically work from a rough stereo mix, which isn't always how you'd prefer to work on your own projects. You'd like to be able to solo/mute tracks and bypass effects and experiment with different mix options as you go. Plus that stereo guide is continually out-of-date as others add their parts. Synchronization can be a problem, requiring you to export every file from 00:00:00 even if the part is two seconds long and only comes in at the 5-minute mark.
SkyTracks tries to address those challenges by creating a virtual project online, which any contributor can work on at any time, without having to download other people's latest files to their respective computers. There is a plugin (VST or AU) for your DAW that lets you work locally and upload your changes without having to manually go to a website and upload the files. In the demonstration at least, it all looked pretty seamless.
It's also a music-sharing and storage service, with unlimited disk space. They also offer a multitrack player that you can embed in your own website for playing the projects.
And it's pretty cheap, too. $3.95 per month. You can try it for free (AFAIK for an unlimited time). The only limitation of a free account is a maximum of two projects. When you open a free account you'll have a demonstration project already there to play with.
[EDIT]
Oh, yeh, and it supports video files, too (MP4 or MOV).