I'm cheap, so, I wear what i have and it's nothing fancy. Layers are important, and don't neglect your ears, hands and feet. I rode to work several years ago when the temp was 7 degrees F. The commute was 14 miles one way and I was fine except for my feet. I haven't had to ride in anything that cold lately, but I am going to buy something to protect my feet from the elements just in case.

You will actually be surprised by how easy it is to stay warm when it gets colder outside.