SpiderOak does zero knowledge backup & sync across Windows, OSX, and various flavours of Linux, and has Android & iOS clients; 2GB free + 1 GB for each friend you refer, or $100/year for 100GB (deduplicated) and no limit on devices.

The UX is not as nice as Dropbox, but I've been using a free account for about 2 years now and I'm pretty happy with the service and the support (especially given that I'm not a paying customer).

Quirks are that deleted files count towards your quota until you delete them from the list of deleted files (deduplication makes this a bit less painful), and there isn't a way to bulk restore files deleted after a certain time.