Studded tires. The studs are actually only really useful on ice; on snow they're pretty superfluous and knobbies would suffice.
I've ridden the 11 miles to work on days after ice storms where it was almost too slippery to walk, and I certainly would not have driven my car.
My ride is over plowed roads, with 4 miles of them gravel that is packed snow surface (pretty slippery) and hilly most of the winter, so I don't need super aggressive tires. I run Nokian Hakkapeliitta 35s:


I got mine from Peter White Cycles. The website is very enjoyable:
http://www.peterwhitecycles.com/

Caveat: Be prepared to work for it. The air is thicker and winds stronger in the winter, and studded tires do not roll as easily as what you're probably used to (assuming you're on road tires now).