Sigma's 18-35mm F1.8 DC HSM ($800) is a formidable piece of glass if you're looking for a medium focal length (short, if you're shooting full frame), wide aperture add-on for your DSLR, provided you've got the budget for an $800 lens.
Terrific image quality; Wide f/1.8 aperture; Snappy autofocus
Short zoom doesn't do much; May be overkill for APS-C users; Flaky lens cap