I've tried a bunch of different fisheyes for pentax digital cameras, and my favorite by far is the
pro optic 8mm It's also marketed under the Bower, Rokinon and most likely other brands as well (Samyang?). In any event it has a full 180degree FOV on 1.5 crop sensors. It is a rectangular projection rather than circular, which I prefer. You can always crop if you want circular. It is a manual focus lens, not a big deal at 8mm, the DOF is HUGE! One of the biggest benefits of this lens is it's very very very good flare resistance. It's kinda hard to eliminate the sun when you've got a 180 FOV! Yet I have yet to have any flare problems with it. I'm at work and don't have access to some sample shots, I'll try to remember to post them tonight.
edit: looks like Lowell beat me to it. But we like exactly the same things about this lens.
NaCl(good crop sensor fisheye, check it out)H2O