Sharon, I think Dropbox is the best cloud service, and I use it for my work-related files. I'm now up to 9.9 free GB, thanks to referrals, up from the default free 2 GB everyone gets. Even so, I have it filled up to 88% or so.
For this reason, I use SugarSync to mirror my Calibre library online. SugarSync gives you 5 GB for free to start with. Thus far, this is enough to handle my Calibre folders.