Amazon offers anyone with an Amazon account 5 GB of free space on its Amazon Cloud Drive. They also provide unlimited free cloud storage for digital music purchased from Amazon, and 5 GB of storage for documents and books that you email to your Kindle reader. Amazon's Cloud Drive offering isn't as robust as some others, and Amazon really pushes it only for image and video storage.
The computer version is a web app (web page in your browser), while the apps for iOS and Android actually are called Amazon Cloud Drive Photos. For Kindle Fire users, Amazon Cloud Drive is already set up and activated. For everyone else, just sign into your Amazon account (or set one up) and then activate it.
Amazon Cloud Drive's biggest limitation on a computer is that it doesn't have an app so that you can treat your cloud storage like any other folder. That makes it not very useful if you want to use it as an active drive for saving and loading documents for word processing or image editing on a computer, for example. But it's OK for backup storage.