I use
Dropbox and am very happy. If you use my link, both you (and I) will get an extra 250 mb free. I recently had a massive hard drive failure and had to upgrade my computer. It took awhile for all the stuff to re-download, but once it did then in two clicks I had all my Calibre library back and all of my iTunes library. The thought of reconstituting my 900-book ebook library from scratch gives me cold sweats. Thank god I had the backup!