Lensbaby has just announced a 5.8mm ƒ/3.5 circular fisheye lens with manual focus for Canon and Nikon mounts. Although it has been optimised for APS-C sensors, you can use it on full-frame cameras. You'll be left with a smaller image circle is all. It should be able to focus as close as ¼", going all the way to infinity, and offers an 185° angle of view. At a smidge under $300, it's an extremely affordable addition to you kitbag, it just relies on you using its focusing ring to get your subject sharp, rather than on your auto-focus. If you're not sure that you can stretch to a Canon or Nikon-made lens, it's an intriguing alternative.
It's not on sale yet, but you can pre-order from Adorama. Canon's here; Nikon's here and both are priced at £299.95!