Sigma EX 24-70 f2.8 Macro DG Lens Test Photographs
The Sigma f2.8 EX 24-70 lens can be bought for much less than the Canon and Nikon equivalents. (Some reviews say that on a crop camera there's nothing to separate the Sigma from the Canon L).
I find it very sharp wide open at 24mm (try clicking the photo of the stuff on the table and view it full size)
Click on any photo to see the full-size image taken directly from the camera together with EXIF data