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My guess you could use InVision. They have a really good sharing/commenting system. You could just set up a folder inside a project that's called "Initial Assets" — and then display all the follow-up work in other folders. Dropbox has also been offering commenting for a while, but I personally haven't had a chance to explore it.
I would suggest using Pixelapse. They have version control, shared workflow, public gallery, commenting, annotations and some other features that can be useful in a shared working environment. Github also supports PSD viewing and diffing. Hope this helps!
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