If you don't mind hosting yourself, Seafile is also open source and has really nice Mac/Windows/Linux/Android/iOS client apps. It also does delta syncs, which I don't believe ownCloud supports yet:
What's with the lack of delta syncs in all these systems? As far as I know, neither Google Drive nor OneDrive do it. Now you're saying ownCloud doesn't do it either. I didn't even think to look at it for oneCloud because obviously they'd support it.
I consider it to be an essential feature, to the point that I won't bother with something that doesn't support it. I guess I'm unusual in that respect.
I suspected I would get downvoted for not loving Dropbox. And I was right. Interesting.
FYI
rdiff is the reference implementation or first example of a utility written with librsync, which is the library that Dropbox used to build their "business".