Dropbox!
Basically, once you've set it up (which is easy), you have a Dropbox folder on your computer. Anything in your Dropbox folder lives on your computer like a normal file, but also on Dropbox's servers. If you set up Dropbox on two computers, your files are on both of them, and Dropbox keeps track of when they change. Update a file on Computer A, and Computer B will have the updated file a couple minutes later. Also, you can go onto the Dropbox website any time, and find all the previous versions of all your files.
At this point, if my hard drive died again, I'd pop in a new one, set up Dropbox, and have lost nothing. Like I said, it's awesome. ^_^