I discovered dropbox through Gizmodo and Lifehacker and I have to say that it's a great tool for a writer who is working on several systems.
What is dropbox? It's a program that allows you to share a folder no matter where you are. I've been working on some writing projects where I usually put my files on a USB key and lug them around from my desktop to my laptop. I try to back everything up locally which is fine except sometimes you edit something locally and forget to replace the file on the usb key. I'm sure a more organized writer would say that it isn't a problem to do this but I have been known to confuse myself as to what draft I'm working on.
Enter dropbox. I install dropbox and get an account and I put my files in there. I install dropbox on my mac laptop and it automatically synchronizes to the dropbox on my pc and vice versa. Forgot my usb key? No problem. The only thing I need is a wifi connection and these are publicly available in the places I go to write (coffee shop or pub). Those without free wifi don't have my business.
In full disclosure, I am no way affiliated with dropbox. I love that it's free and it's been a great addition to my workflow. I recommend it.