The Sigma 10-20mm (I bought is second hand for $320 in 2008) is a very good one. Very low barrel distortion even @ 10mm and a FOV of 97 degrees over the long side.
An alternative is the newer 8-16mm (even wider angle !).
The site
www.photozone.de has an excellent set of review of lenses.
Another site is
http://www.dpreview.com/products/lenses with test reports.
Another option is making more pictures and stitching them.
Hugin is a good auto stitching tool.