I have never understood the fasination with fish eye lenses; but I realise a great many people love to experiment. Or rather would like to experiment if dedicated lenses were not so expensive.

This full and complete article describes how a commodity door peephole can be used as a fisheye converter lens, thus bringing the total parts cost of a serviceable digital camera with a fisheye lens to under US$20.  The paper, also available as a PDF download, covers everything you need to know from which camera the techniques were tested on, though to noise reduction. They also explain how stills and video can be captured via a USB connection and the software required.

Image quality is not great and is recommended only for prints around one inch in size. The sensor's limited dynamic range is reduced because the camera's autoexposure logic is fooled by the dark area around the image circle, making it virtually impossible to avoid overexposed highlights in bright, high-contrast, lighting. But it looks like a fun, inexpensive project.