Bitcasa starts you off with 10GB for free, but $100/yr throws the floodgates open and gets you unlimited storage for anything you want to sync, share, or back up. Plus, you can connect as many computers or devices to your account as you choose, so you don't pay by the PC. It's client isn't quite as robust as some of the other tools here, but if you're more interested in a flat fee for unlimited space and you can handle the details of which files go where and when, it's a solid option. You can read more about Bitcasa's pricing here.
You can either tear out your hair when a disaster strikes your hard drive or you can prepare for it ahead of time, but data loss is as inevitable as death and taxes. An online backup service is one of the best ways to protect yourself against such threats as a crashed hard drive or accidental deletion. Natural disasters such as fires, floods, and earthquakes can also spell the end of your digital media and documents. Even if you're among the very few who diligently perform local backups at regular intervals, you could still lose data if you don't store backups offsite.
Backup services vary widely in how they set up and perform backups. For example, the totally hands-free Backblaze automatically encrypts and uploads all your important files without any input. On the other hand, services such as IDrive and Acronis True Image let you choose specific files you want from a file tree. Note that some services restrict you from backing up specific file types or using particular sources, such as from an external or network drive. Make sure the service you choose supports all your needs.