[Pixelapse](http://www.pixelapse.com) comes pretty close.  

Here's what they offer:

* A public gallery option
* Downloadable source (e.g., PSD, AI, etc.)
* Revision history
* Commenting and annotations
* Dropbox-like saving behavior

Here's what they don't have:

* Licensing information for public works
* Explicit "fork" behavior and "pull" requests