The super-wide APS-C lenses (Sigma 10-20, Canon 10-22) are pretty good and give an equivalent FoV of 16mm (or 15mm on the D300). There aren't many rectilinear lenses wider than 16mm for full frame. Stepheno has borrowed my (full frame) Sigma 12-24 to use with his 5D and the results at 12mm are quite impressive, but I'm not sure how often you'd use it once the novelty wears off.
