SpiderOak is well known as one of the most privacy-centric cloud storage services, but it's also a great backup service. The same power and features that you get for file syncing and access extend to its backup client, and SpiderOak's "Zero Knowledge" policy extends to your backups as well. Even they don't know what you're storing on their servers, and all of your data is encrypted on their servers and before it leaves your computer. The SpiderOak desktop client has a fully-featured backup tool in it that lets you back up your entire desktop, documents, email, music, or movies right to the cloud, or you can hit the "Advanced" tab and pick the files and folders you want to save, including external drives, network drives, or anything else on your computer. As you add files to your backup job, you'll get a live preview of how much space you'll use with what kinds of files, and whether you have space for it. SpiderOak supports Windows, OS X, and Linux, has mobile apps for iOS and Android, can do incrementals, and if you uncheck a file to stop backing it up, the files will still live in your SpiderOak account, just as an archive. Your backups happen in the background, or when you schedule them.
Bitcasa Infinite Drive is relatively new, but it's one of your favorite cloud storage providers in general, mostly because they offer virtually unlimited stroage for syncing and backups. When we say unlimited, we mean it—some of you are using terabytes of storage with Bitcasa. It's not primarily a backup service though, and while it was built for file syncing and storage, the Bitcasa desktop client does support regular file backups. Bitcasa supports Windows and OS X, and encrypts all of your files before uploading so they stay safe from prying eyes. Bitcasa even keeps revision history, so if you've backed up a file multiple times and need an older version, you can pick it out and restore it. Plus, you can use the Bitcasa mobile apps for iOS, Android, and Windows Phone to access your data on the go.