I got myself a little 2TB external USB 3.0 hard drive ($99! Past Sean, be very jealous) and wrote a backup script; my first version had all of this complex Towers of Hanoi rotation scheme, but then I realized I didn't need any of it if I used btrfs's snapshots. Now, my backup script consists mainly of:
sg_start --start /dev/sdh # Tell the drive to spin up
mount /dev/sdh1 /mnt/backups
btrfsctl -s /mnt/backups/backup-$todays_date \
rsync -va --numeric-ids --delete-before --ignore-errors \
--partial --inplace $backup_paths \
sg_start --stop /dev/sdh
It does a little more than that (error checking, logging, etc), but not much more. After the first backup, the disk usage was 15GB; ten days later, I have ten incremental backups, and it's still 15GB.
Of course, the problem with this is that snapshots are, essentially, COW hard links; this means that if there's a corruption on the disk for a file, it'll affect all child snapshots. My mitigation is to add another backup disk: they're only $99 (cripes, I can't get over that price), and each backup is taking about 8 minutes to run (most of that time is spent in rsync, detecting changes) -- I can easily affordable to run two backups a night. It's not as safe as a rotation backup, but it's safe enough.