The states of the former U.S.S.R.
Women traveling to Russia and the states of the former U.S.S.R. may not encounter the same aggressive cat-calling they might find in the Mediterranean, but other safety issues warrant precaution. Public transportation needs a special category all its own in this respect, and is probably the one place American women need to exercise the most caution. While living in Ukraine and traveling throughout the region, I heard horror stories about attempted sexual assaults on trains, and was the victim of more unwanted touching than I care to remember.
Throughout the states of the former U.S.S.R., the smile most Americans effortlessly put forth may cause you trouble. Culturally, the smile in former Soviet countries has come to mean that you're sly, crazy, or you're interested in the person at whom you're smiling. This may stem from the repressive nature of the former Soviet regime. It's hard to look stoic and indifferent, but I recommend practicing your best emotionless face in preparation for a trip to the region.
Lastly, a trick that sometimes works to discourage unwanted attention is slipping a simple gold band on your right ring finger. Putting it on your left hand, like in the U.S., will only signify widowhood, and hence availability. Whenever I saw men approaching me on the trains, I turned my grandmother's ring around so that only the band showed and pretended that I was married.