I have a pair of gore thermal shoe covers that I use from around 40-55°F. I have a pair of diadora polaris 2's that I got from the closeout bin a couple of years ago. I got them a size too big, and they are comfortable with heavy socks down to about 20°F, maybe a little colder. Below that I wear boots with platform pedals. I don't ride my fixed-gear when it's below freezing.