hdup is used to back up a filesystem. Features include encryption of the archive (mcrypt/GnuPG), compression of the archive (bzip2/gzip/lzop/none), the ability to transfer the archive to a remote host or restoring from a remote host (with SSH), the ability to split up archives, and no obscure archive format (it is a normal compressed tar file).
rdup is a utility inspired by rsync and the Plan9 way of doing backups. rdup itself does not backup anything; it only prints a list of the names of files that have changed since the last backup. It also handles files that are removed, allowing for correct incremental backups. Auxilary script that implement a backup strategy are included.
Re: How is this different from find?
> find . -type f -newer .lastbackup
what this doesn't take into account is removed files, rdup
does show files that are removed since the last run.