For years I've been looking for an easy way to work on the same document on different computers. I've never been good at copying work I've revised from one computer to another; when I do, I often forget which computer I last used. This logistical dilemma has, on more than one occasion, discouraged me enough to stop me from writing.
Dropbox has ended my search for a solution—a perfectly executed, elegant little program that automatically updates any file I ask it to across computers, as I write. And it's free.