Microsoft has doubled its 15GB cloud storage space for free users if they activate the "Camera Backup" feature in its OneDrive app for Android or iOS before the end of September. They just have to activate it once for getting the bonus space. The feature can be disabled immediately after you get your free 15GB.
Keep in mind that when you enable "Camera Backup" feature in OneDrive app, it says that you will be 3GB of bonus space. After enabling the feature, go to "Manage Storage" page to verify if you have received your 15GB "Camera roll bonus".
OneDrive user now have 30GB of free cloud storage (40GB if you signed up years ago), which is double the free storage of Google Drive, three times the storage of Box.com, and about 4 times that of Dropbox's free version, which provides only 2GB base and 5GB camera roll bonus.