No, not those kind of fish eyes!
I mean a fisheye lens, which on a crop body, is pretty hard to get a hold of because you needs a focal length of 12mm or less to really get a decent field of view. Fisheye lenses are a bit of a trick lens, not something you'd want to take as your only lens to a family reunion because it'll make Uncle Bob's huge nose look even bigger. But, used appropriately, it can get images you can't get any other way, including indoor shots showing the full room (see here!) and specialty shots (here, here, here, and here). Fisheyes are also often used for scientific applications where a large field of view is required.
Note the difference between a circular fisheye (which will render the image as a circle of the frame with black around it) and a full-frame fisheye (which produces the same distortion, but covers most or all of the frame with the image). Many modern lenses with wide angles try to avoid the fish eye distortion as much as possible, but even if they don't, it is easy to remove the distortion in software (see how here).
For Canon, the options are:
- A cheap lens attachment. Generally, these are diopters that fits on the end of your existing lens and give you a true circular fish eye. Range in price from $30-$150 (for instance, this one is $35) but I don't expect much from the optical quality. Although, I have seen some nice images using an Opteka fisheye (and a mini-review!). The images are typically really soft on the edges, but it is almost worth buying to just play with it.
- A quality prime. Currently the Sigma 8mm f/3.5 is the only game in Canon town for an autofocus circular fisheye with a focal length smaller than 10mm. From what I have heard it is a great lens, but $600+ is a lot for something I won't use much.
- A quality zoom. Surprisingly, these are cheaper than the Sigma prime, but they also aren't as wide. A lot of the expense of a wide angle lens is from the glassed needed to get the wide end.
- Previous generation Nikon or Canon lenses -- expensive and super rare because 8-10mm focal lengths are extremely special purpose with a full-frame lens. Not practical at all (but I did look around!).
- The Russian (Belarussian, actually) Peleng 8mm f/3.5. This is a surprising little gem for a completely manual lens (no autofocus or auto-aperture) available for a little under $300 on eBay. Well, good for a Russian import lens at least (great image quality, reasonable workmanship but it does it's job... like a tank). See Bert Stephani's review here, Jarnell.com's review here, and Marco Pauck's review here. DPC's lens page is here.
Edit: I've added a listing of Pelengs on eBay in case you are interested: