For general cloud-based backup, Dropbox remains our go-to favourite, but specialised applications can benefit from specialised BackupMyTree is a service aimed at keeping copies of your family tree data and making that data viewable from any browser.
Full-scale family tree research involves considerable time and effort, so having an online backup in place for that data makes sense (though I'd hope anyone doing that kind of work already had a full-scale backup system in place for their system).
Like Dropbox, BackupMyTree automates the process of backup for any files it recognises as being in family tree format, ensuring you'll have a current copy online. It also keeps archive copies of the previous 25 versions, which can be useful if your files become corrupted. It supports the GEDCOM standard, as well as files from Family Tree Maker, Family Tree Builder, PAF, Legacy Family Tree, RootsMagic and Family Tree Legends.
BackupMyTree is free to use; the client software is Windows only, though you can also upload files of your choice via a browser.