iCloud only offers up to 5GB of free storage. After that, you can get another 5GB free using Amazon Cloud Drive. If you're a Prime user, however, you'll get unlimited free photo storage (not videos, audio, or documents).
Google Drive gives 15GB to its users, but most of this is usually taken up by your email. If you have a Chrome OS device, you'll get another 100GB for free.
Once you've used up all those options, there's Box, which gives 10GB of free storage, and Dropbox, which gives 2GB.