Launched back in 2008, Dropbox is one of the biggest names in online storage. The easiest way to use it is by installing it on your machine/device, which allows for automatic backups via its mobile apps or (of folders) on desktop.
What you get for free:
With Dropbox Basic account you get 2GB free, though this can be boosted by referring others to Dropbox. For every person who signs up via your referral link, you'll both get an extra 500MB, up to a maximum of 16GB.
iOS, Android, Windows Phone, OS X and Windows. Full details on how to set up BT Cloud, or see BT Cloud FAQs.