It doesn't get below freezing where I live so I haven't had to contend with this personally, but when my friend lived in Boston and cycled throughout the winter, he got the Finnish studded snow tires for his bike:

http://www.icebike.org/the-ultimate-guide-to-winter-bike-tir...

He said it made him feel a lot safer. (That page mentions a concern that there are failure cases related to inappropriate tire pressure that may reduce traction compared to regular tires, but it seems like they normally increase traction by a huge amount.)