Dropbox will soon be adding support on both Windows and OS X for placeholder files that create a full view of your cloud-synced files, even if they're not available locally.
OneDrive (or rather SkyDrive, as it was called then) in Windows 8.1 was a significant step forward in improving the cloud storage experience for desktop users thanks to its novel handling of cloud-synced files. Within Explorer and at the command prompt, every file stored on OneDrive was shown, even if it wasn't synced locally. Double-clicking a file (or using File... Open within an application) would automatically download it so that it could be read and edited as normal.
This system provided a great increase in usability, especially on machines with limited local storage. Instead of requiring you to pick and choose which files or folders to sync manually in order to avoid filling the local disk, you could see all your files and folders in your OneDrive folder. Only the ones that you actually opened locally would occupy their full size; everything else was shrunk to a few bytes of metadata.