Dropbox Project Infinite
This feature is implemented at a low level, and works on the command line.<p>For example if you have a directory that is all stored in the cloud you can `cd` to it without any network delay, you can do `ls -lh` and see a list with real sizes without a delay (e.g., see that an ISO is 650 MB), and you can do `du -sh` and see that all the files are taking up zero space.<p>If you open a file in that directory, it will open, even from command line, then do `du -sh` and see that that file is now taking up space, while all the others in the directory are not.<p>You can right-click to pin files and directories to be stored locally, and right-click to send them back to the cloud so they don't take up space.<p>This is actually very different than traditional network file systems like SMB, NFS, WebDAV, and SSHFS. With a normal network file system over the WAN you would have major latency problems trying to `cd` and `ls` the remote file system. Most of them also don't have any ability to cache files locally when offline, or the ability to manually select which files are stored locally and which are remote. AFS does have some similar capabilities.