I've been using Dropbox for over a year now, and it has become one of my "can't live without" programs. Dropbox creates a folder on your computer, which is automatically backed up by the server. Even more importantly Dropbox syncs the folder on every computer that you install it on. In other words, you can save a file at work, and it will automatically be available at home. You never have to e-mail a file to yourself again!
I highly recommend Dropbox for anyone who works on more than one computer!
Perhaps the biggest recommendation for it I can give, is that I'm approaching the limit of the 2GB free storage that is the standard version. Dropbox will increase my storage if people click on this link, and install the program! So while this isn't an interest free recommendation, those of you who enjoy this blog, can feel free to support me in this manner.