Over at Instructables, user Banjomaster shows how to make a fisheye lens for his Nikon D90 for just $US16, with the help of one of those wide-angle doorway peephole lenses.
It looks like a pretty simple mod, both in materials required and construction method: It only needs a couple pieces of particle board, the aforementioned peephole lens, the spare lens shield that came with his camera, and of course some duct tape. The only possible caveat: The replacement wide-angle lens is significantly smaller than the camera's lens, so there's a circle around all of the shots. On the other hand, it's sort of cool; it makes everything look like you shot it through a hotel door. Check it out, we're sure it can be modified for other makes and models of DSLR. [Instructables]