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.
Box is a cloud storage whose initial market was serving business and industries, and only recently has pursued the personal cloud services market. Box offers 10 GB of free file storage with its "personal" service, and syncs across all your devices.
OneDrive is Microsoft's cloud storage service, recently renamed from SkyDrive. It has a generous free level, offering the same 15GB free as Google Drive, plus 3GB extra if you sync your photos from your mobile device with it, and another 500MB per referral. If you use Microsoft Office Online or OneNote, your OneDrive is the default place that your files are saved.
Dropbox is a widely-used cloud storage service that works on every system via an app or dedicated program, or via a web interface. It is easy to master, and blends seamlessly with your computer file system. It's really useful you use different devices, and want to move your files from one to another. It's also handy when you want to share files with friends and family members, or even make a file available for the public to download.
Evernote is an organizer app that lets you put your notes, to-do lists, outlines, photos, and web "clips" into notebooks. It runs on every platform, and since everything is saved automatically "to the cloud," you can write on your tablet or computer, and then access from your smartphone, if you wish. Essential for students, but great for everyone.