Microsoft have re-branded their SkyDrive Cloud storage to OneDrive, and to coincide with the name change they are offering up to 8GB of bonus space - 3GB if you enable Camera Upload from a smartphone app, and 5GB in referrals (500MB increments).
New sign ups get 7GB as standard, so this can max out at 15GB of free storage. If you have a "grandfather" account, you will have 25GB to begin with, so this can be expanded to 33GB - for free.
I have two Microsoft accounts and to test I signed into Account 2 using a referral link from Account 1 - this was enough activate the bonus 500MB for each account!
Yes, you can get more free storage from providers like Box and Mega, but OneDrive comes baked into Windows 8.1, Xbox One and Windows Phone - so the more free space available, the better. Dropbox offer a similar referral scheme but the process requires the other party to install the app on a new machine - here, you get the bonus when the second person signs in to their accounts