SparkleShare is an open source Dropbox-like software with clients for Linux, Mac, Windows, Android. As you can see in the screenshot below, the client is very neat (Windows/Mac/Linux).
It uses Git as a backend but is extensible to other backends. How about making an Alfresco backend? We could use CMIS (or WebDAV) instead of Git. Clients for all platforms are based on the same source code (Mono), so we would just have to implement the protocol layer once, and we do not have to care much about things like GUI.
That would make Alfresco much more attractive for users, with the ability to use their files without an Internet connection. Any volunteers?