I actually use two Gator products. These are their neoprene road booties (they also make a mtb version):
http://www.gatorgear.biz/robode.html

And these are their neoprene socks:
http://www.gatorgear.biz/basicsocks.html

The booties are for most extreme conditions and the socks work great in the rain, or you can wear both when it is REALLY cold.