Most people are familiar with Dropbox. I thought it was worth mentioning however, as this is where I like to back things up so I can access them later if something should happen. It’s my “just in case” contingency plan.
I like to back up my manuscripts to my dropbox. I find it a great way to ensure they are safe, and it is just one of the many ways I choose to back them up. I also have a habit of emailing them to myself in my gmail account. Emailing them serves two purposes. One as a backup, and the other for a date stamp of when I wrote it. That is handy for copyright purposes should I need them.
In any case, if you aren’t backing up your work somewhere, I highly suggest you open a free dropbox account. Computers fail. Flash drives fail. External hard drives fail. Having your manuscripts safely in cloud services is just one more layer of protection if the worst should happen. I figure it is better to be safe than sorry.
What do you use to back things up?