SpiderOak's beauty is that it's a combination cloud syncing and storage service as well as a backup client all in one. Unlike some of the other services though, you'll have to pay for the storage you use. You get 2GB for free just for signing up, and you can get up to 10GB by referring friends. SpiderOak Plus nets you 100GB for $10/mo to use for syncing and backups, and every 100GB after that is another $10/mo. Plus, you can connect as many computers to any SpiderOak account as you want, so you're not paying by the system. You can read more about SpiderOak's plans here.
Many services also offer a feature called versioning. This saves incremental changes you make to files as recoverable snapshots of the file. It's useful in case you need to get back information from an earlier version or if your latest file save becomes corrupted. Services vary widely in how many versions they keep and how long they're saved. Best-in-class services, such as SOS Online Backup and SpiderOak ONE keep an unlimited number of file versions forever.
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.