I've never used it, but
Copy works for pretty much every popular platform out there at a price.
Spider Oak is free, but you only get 2GB.
I can vouch for copy. Its awesome, they gave you 5Gbs extra for each referral, if you share a folder with someone the total space used is divided between you both so you each get half of it counting to your total instead of you getting the whole thing.
Damn, since I moved to my new pc (i7, 16Gb, gtx760, z97 mobo) I have never experienced so many issues on linux.
Rigth now, its totally unusable. Usually there is a couple of fixes to be done but this time it wont even boot. It gets stuck on loading the kernel into ram and I dont even get an error message.
Freaking thing will happen for 3 days after a week of no problems, then fix itself (or reinstall, look! linux is catching up to windows!) then stay again for a week and broke again.
Damn, what I bad luck this time. Sound does not work unless I install kernel 3.18. But then Nvidia drivers wont work in 3.18 because they broke them. Wireless wont work unless I compile a kernel mod. Which breaks every 2 to 3 hours.
Ugh. It reminds me of 2000 or so, nothing but problems. Boot windows, install linux on a vm. Full screen in both monitors. Fixed.