Pick&Zip lets you download any Facebook album as a ZIP or PDF
Now, Pick&Zip is by no means the only app that lets you backup Facebook photos, but it's definitely the most lightweight and simple tool I've tried. All you do is sign in, browse albums or people, and download the photos. You can either download the photos as a ZIP or as a pre-compiled PDF. I'd recommend the ZIP option because the PDF seems to pointlessly up-scale your photos, exacerbating the poor JPEG quality of images on Facebook.
Seriously though, why doesn't Facebook just offer a 'backup my photos' button? In a world where almost no one prints hard copy photos and scant few keep proper digital backups, it's the least that Facebook could do!