I thought I'd share this because I think it's a pretty good deal. I own a WD My Cloud EX2 which comes with Elephant Drive preinstalled. You can can backup 1TB to ElephantDrive for $10 per month or $99 per year. It comes with a really easy to use interface so you can manage which folders are backed up.
I know some services like CrashPlan provide unlimited backups but installing it on the My Cloud was too much of a hassle.