As always, a cheap one that performs well, and (shock, horror) an auto focus lens.
May I present the Vivitar 19-35mm (sold under many other brand names but made by Cosina). Users of crop sensor camera probably will have littel use of this lens, but I bought the lens as it works on full frame/35mm cameras and is MUCH lighter than the 24-70mm. It's primary use will be on film cameras and/or urban architecture. It's not bad in the centre wide open and is usable at F5.6. Stopped down to F8 or F11 it works quite well (it only cost £40 with both lens caps and a hood). Plus, ultra wide lenses can be good for creating perspective distortion.
Click the images for bigger sizes (jump to Flickr:
Click to read the rest of this post and/or make comments