I use Dropbox very regularly and they recently have started a new promotion where in order to get more people using their service they are offering free storage to new and existing users in different ways, all meant to get their name out there and more people using the service. I saw it first on an old professors Facebook page. For each referral he sent to Dropbox, they would give both him and the new referral 500MB (half a gigabyte) additional space for free. (Up to 16GB)
Beyond the obvious upgrading to a pro account ($99/year for 100GB) there are different ways to get free storage. You can refer people to Dropbox by sharing a link. (Like I am hopefully doing with any of the links I pepper throughout this article. ;) You can connect your account to Facebook, (+125MB) and or follow them on Twitter. (+125MB) If you tweet about them you can snag an additional 125MB!
Dropbox is a cool service that basically makes a folder on any computer or device you install it on that is shared with whoever you want to share it with. You can just share with yourself and have multiple computers connected, whenever you drop something in the folder it is synced to all the folders across machines on the account. Here is a tour to get you started with cloud sharing. Dropbox explained.
I use it for my website files for any of the sites I maintain, and sharing files with my brother who lives in Maryland. We even have a specific folder set up for my mom so she can easily see and get pictures of her grandchild whenever he drops them in there. What could you use free online storage for?