I had an old DVD player in my car, and I broke it open, and dug out one of the lenses. There are two lenses, a curvy one, and a flatter one. You want the flatter of the two.
I took the lens, drilled a hole in an old Maker Faire badge, and cemented it in. The lens is just above the "2010."
The results can be really cool.
You do have to tinker with it a lot to get a good picture-- if the lens is off-center everything gets messed up, you have to be careful to focus it properly, and of course there's huge vignetting-- but that's part of the pleasure of it.
Now on to making a pinhole lens for my Nikon D3100!