I love Dropbox. I use it to back up and synchronize all my important files, and it has pretty much replaced my need for external hard drives and USB keys. I also use it to share specific folders with co-authors or students so that any changes they make or files they add are synchronized automatically across everyone’s computer. I also don’t have to worry about updating all the files on my laptop before traveling — as long as I will have an internet connection while away, all my files will be updated.
If you haven’t tried Dropbox yet, you really should. And, if you decide to, go ahead and use this referral link — it will give both of us some bonus storage space free.