GmailSync - Automated backup solution using Gmail
Blogger Nauman Leghari has compiled a very useful tool that automates backup to synchronize files on your computer to your gmail online storage. You can use this as your primary file storage.
How Space is determined -
Space in your Gmail account * # of invites you currently have in your Gmail account.
This method requires that you edit a config file that matches with your Gmail account setup. It is a relatively easy method to automate backups.
GmailSync - Free automated backup solution using Gmail