I saw this cool post: http://photojojo.com/content/diy/make-your-own-fisheye-lens/
And decided that I could do one better: I would go to an eyeglasses shop and get the highest prescription lens-blank they had.
A digital camera (SLR or point-and-shoot)
If it's an SLR, you also need an 18-55 lens, stock with most cameras.
Very thick, uncut eyeglass-lens. ($15)