Here's a Perl script to back up a flickr photo set:

#!/usr/bin/perl -w

use Data::Dumper;
use Flickr::Photoset;
use Flickr::Photo;
use LWP::Simple;
use strict;

my $params = { api_key => 'your api key'};
my $info = {};

my $photoset = Flickr::Photoset->new($params);

# specify a photoset
if ($photoset->id({id => '72057594072478931'})) {
  my $title = $photoset->title;
  my $owner = $photoset->owner->real_name;
  my $photos = $photoset->photos;
  foreach my $p ( @$photos ) {
  my $id = $p->id;
  my $sizes = $p->sizes;
    foreach my $s (@$sizes) {
      if ( $s->{'label'} eq 'Original') {
        $info->{$id} = {
          source => $s->{'source'},
          title  => $p->title,
          server => $p->server
        };
        my $ret = getstore(
                    $s->{'source'},
                    $p->title.'_'.$id.'.jpg'
                  );
        print 'response was '.$ret.' for '.$p->title."/n";
      }
    }
  }
}

Download an entire directory tree:

$ wget -r ftp://username:password@site/path/to/suck

If you need to upload an entire directory structure, check out wput on sourceforge.net. It works the same way as wget only in the other direction (i.e., supporting various protocols).

http://wput.sourceforge.net/

Thanks to Aronalle for this tip!