Here is a great twenty dollar DIY fish eye lens for Canon EF mount cameras.
Bhautik Joshi came up with this novel idea for a cheap solution to satisfy his fish eye addiction. By using a peephole lens and some other cheap materials, he has outlined the steps to make this lens on his blog. This and other DIY projects can be found on his site. Here is the list of materials for the project.
▪ Projector lens (US$3, second-hand)
▪ Door peephole lens (US$6, new from the hardware store)
▪ T-mount to EOS adapter (US$9 new from eBay, but you can use a drilled out body cap instead which would be ~US$4)
▪ Soda can (US$1)
▪ Packing foam (US$0)
▪ Cardboard stopper rings (US$0)
▪ Plumbing clip (US$0.5)