Besides Dropbox and Google Drive, I just realised that Copy is also interesting. They provide 15GB for free online storage and, in order to celebrate the launching, give additional 5GB for each completed referral. Yesterday, Gangsar Kurniawan invited me to use this service, and when I already signed up and installed it on my computer, both of us earn an extra 5GB. Moreover, just like other service, Copy is also compatible with my smartphone.
On the other hand, Dropbox just provides free online storage starting at 2GB and could be increased to 18GB using referral system, where we will get additional 500MB per referral. While Google Drive gives us free storage for “only” 15GB. Unfortunately they do not have the referral system, so we have to sacrifice extra amount of money if we want to increase the storage.