Well, I was going to wimp out and take the bus half way the other day when it was -5F (actual, not counting the windchill) .... Well, plans changed when I got to the bus and the bike rack was frozen solid!
So... I just rode the 14 miles. It wasn't too bad, though my feet were pretty cold by the time I got to work.
-12F is my personal record... I've ridden twice at that temp, once in high school and once last winter.
This is what I wore:
Thermal socks ($1 from Walmart sporting goods section) over my normal socks, with a pair of Brahma boots (I like the boots because they're water proof)
Medium weight long underwear (<$10 from Target)
Blue jeans or Military issue Cammo pants
Athletic works wicking under shirt (<$10 from Walmart)
Sweat shirt
Zippered hoodie sweat shirt
Water resistant windbreaker ($10 from Target)
waterproof, windproof hunting gloves ($10 from Walmart sporting goods section)
Athletic Works neck gaiter ($3 from Walmart)
Stocking cap for extra warmth on my ears
If it's above 10F I drop the stocking cap and use a ball cap to keep the hoodie out of my eyes.
Above 20F I drop the long underwear
Above 30F and I can wear a t-shirt instead of the inner sweatshirt
I also have a pair of windproof, water resistant pants I use if it's raining or snowing. they fit over whatever pants I'm wearing for the temps.