As an alternative I have never seen Powerfolder (http://www.powerfolder.com) mentioned here, and that's a pity. Powerfolder surely is not a nice and shiny start up (German private company, since 2003), but it has worked for us for over 3 or 4 years now. As I understand Powerfolder originally was a client to just sync files and folders across computers but they added "cloud storage" a few years ago, so your devices need not be connected at the same time. The Java client works an Windows, Mac and Linux, it's "functionally" designed, to say it politely (no shiny icons) You can sync backup or share any file or folder on your computer. I've tested nearly all sync and backup services but none of them made me switch, Powerfolder's sync speed and reliability is just superior (they can LAN sync too). And heck if you don't trust "the cloud" you can just buy their server software and set it up on your own hardware.

I still use Dropbox for easy private file sharing, just not professionally.

[Edit: ,just NOT professionally]