Dropbox having good Linux support is a big reason I use them personally and professionally (both paid). But geez do they mess other things up. For example you can only share top level folders. We have to share with various outside parties and that limitation leads to a very cluttered file structure. The solution for multiple accounts is beyond annoying. The correct way of doing it how google does it - you provide how…
AeroFS (what I use), Bittorrent Sync and Syncthing (Golang, OSS) all support Linux (and Mac/Win) and are good alternatives.