The page says 'Name your price' but the minimum is US$20.00.
There is a free version but the pro version offers:
- Link all your devices
- Manage permissions on the fly
- Save space with Selective Sync
- Control bandwidth usage
- One time send
For more information see:
I have been using this piece of software to keep my files sync'd between many computers/raspberry pi's for a while now - mainly because I like having control of my stuff and not have it in the cloud. The main drawback is that to sync, you have to have the other computer(s) on but I've gotten around that by having an always on Raspberry Pi. Other than that, it's brilliant.
There are also mobile apps which are free and I can view my files on the run whenever I want.