I have a Samyang fisheye which is designed for crop sensors, but I've used it in my 5d I. I think you end up with about 8MP after you crop away the dark bits, but this would be bigger on a 5D II obviously.
It's a great lens for around £200, very sharp. MF only but I don't find it much of a problem as the DOF is so big on such a wide lens.