Swallowtail butterfly; photo by moonbird (Creative Commons licensed)
PZ posted the picture above, and asked his readers for information on what it was. It was quickly identified as a gynandromorph, a butterfly that is literally half male and half female. This page has even more pictures, and explains the biological basis of the phenomenon in layman's terms (the gynandromorph results from an error during the first cell division of the butterfly zygote that causes half of the butterfly to lose its male-determining chromosome).
[Update 9/23/06: A colleague sent me a link to a page on chromosomal mosaicism, which has more details on the developmental biology behind gynandromorphs and similar genetic errors.]