Déjà Dup is a simple backup tool. It hides the complexity of doing backups the Right Way (encrypted, off-site, and regularly) and uses Duplicity as the backend.
rdiff-backup backs up one directory to another, possibly over a network. The target directory ends up a copy of the source directory, but extra reverse diffs are stored in a special subdirectory of that target directory, so you can still recover files lost some time ago.
Duplicity is a software suite that provides encrypted, digitally signed, versioned, remote backup of files requiring little of the remote server.