Using multiple cloud storage services means you're less likely to run out of potential backup space. Until November 23, Encrypted storage provider Tresorit is offering up to 25GB of storage for nothing.
You score 5GB for signing up, an additional 5GB for downloading the Android or iOS clients, and 3GB per friend you refer (up to a maximum of five). That's a relatively generous offering compared to obvious rivals such as Dropbox (though there are plenty of ways to score extra free Dropbox storage). Hit the site for more details.