CloudBacko seems like an interesting option. It combines multiple free cloud file sync services to give you "unlimited" space. The only downside to it is that you may be violating TOS agreements using the services this way, and at any point they could disable your account. Losing your backups in the process.
Crashplan, Carbonite, and Backblaze all offer unlimited backup plans for very cheap. The $5 a month is more than worth it to have a little assurance that I can actually recover my backups.
Cloud backup is still pretty new. And there's no telling if the business plans can survive larger and larger backups from users on their unlimited plans. So even if you're using one of the paid services, I would suggest you backup to at least more than one location (either on-site, or a second cloud backup).