Poladroid.

I recently discovered Poladroid (for both Mac & Windows). It turns JPEGs into "polaroids" that can then be saved, emailed, whatever. And it's free! It has some tongue-in-cheek nostalgic features as well: the pictures take 3 minutes to "develop" (though shaking them speeds this up), and you can only make 10 pictures in a session, like a Polaroid cartridge. Resulting files are 400 dpi, so they look pretty good! The processing is randomized to a degree so each picture is unique.