There are a dozen more cloud storage options, but only three warrant most people's attention.
One is Microsoft's free SkyDrive app and tiered-rate service, mainly because it comes with Office 2013, Windows 8, and Office Web Apps as a default storage location, so it lets Windows-centered users stay connected. Only a few apps support SkyDrive directly, so you need the app to transfer files.
Although Google's popular Drive storage service is supported by many apps directly, having the Google Drive app is still a good idea.
If you use multiple cloud storage services, AppSense's free DataNow app lets you move files among them easily from one console. It supports Dropbox, SkyDrive, Google Drive, FTP servers, and WebDAV servers.