Well, if you want wider than 18mm, you have a couple choices. Sigma has a 15-30mm that a lot of people like. Canon has a 17-40mm f/4 L that runs $700ish I think. There's also the 16-35mm f/2.8 L, which runs $1400ish.
Another option is to get the 15mm fisheye from Canon, and then use software to correct it to a rectilinear perspective. This results in a very wide field of view, but it's another task to perform on each image you take with it.
Oh, there's also the 14mm f/2.8 L, but it costs $1700, and seems to be pretty crappy.