First, if you don't have Dropbox setup, you're missing out, get it here for free. Second, this deal will probably end soon but if you go to the forums and get the experimental build, you can earn a free 5GB on top of the 2GB you get already, if you upload 5GB of photos or videos from some sort of camera or from a memory card out of a camera. I finished this successfully today, great deal. But even the 2GB is enough to sync various documents and such between computers and devices. Dropbox can also serve as an offsite backup however beware that if you delete a file in dropbox on your computer it gets deleted on the dropbox server as well, so it's really not as good as a true cloud backup service. I also wouldn't recommend trusting putting highly confidential things there, like anything that could be used for identity theft or financial fraud.
I have used drop box a lot of times before when I used to work online, it is the place where my boss sends files that I could work on and in return I also submit my finished output there too. It is a useful tool online that we could get for free.
Indeed dropbox is very useful especially for me that usually work online, I can easily store my files and easily access where ever I go as par as there is internet.