I recently got one of these to goof around with ($25 anywhere):
It's a little tricky to use and has an f-stop of around f/22ish. The camera reads it as f/00 and doesn't recognize a lens is attached. It's just a plastic mount with no fancy electronics. Also say goodbye to a useful viewfinder with this attached; you can barely see anything you're shooting. It has 4 manual focus modes: 'Single person', 'Multiple People', 'Huge group of people', and 'Mountains'. I'm using it on a Canon Rebel T3i.
Samples:
ISO-100, f/?, 1/20
Holga by
Ryan Dravitz, on Flickr
ISO-100, f/?, 1/40
Holga by
Ryan Dravitz, on Flickr
ISO-100, f/?, 1/15
Holga by
Ryan Dravitz, on Flickr
ISO-100, f/?, 1/20
Holga by
Ryan Dravitz, on Flickr