I don’t really use online applications for my writing, just Word and Scrivener. So I have two backups: 1) a periodic backup to an external hard drive; 2) Carbonite for all documents.
This was HUGELY useful for me last December. I had an ice tea spill on my computer, which killed it entirely. I’d have lost several weeks’ worth of work otherwise (I really do need to do the external backup more frequently…), but with the external hard drive was able to recover all my installed programs, and with Carbonite was able to recover all of my documents up to an hour before the spill.