Wool baby! I'm riding in wool socks for the first time this winter and it makes a huge difference. Just a single layer and over my shoes I wear Performance Neoprene Booties. My commute is 11 miles each way and temps have been consistently in the mid to upper 20s for the past month. Roughly 40-45 minutes of ride time.

One other thing I do is wear a slightly larger pair of cycling shoes in winter to allow breatheability (sp?) and maintain good circulation. That's key. Piling on layers doesn't do a thing if you cut off the blood flow. I actually have 4 different sizes of the same shoe: 42, 42.5, 43 and 44. (I get 'em used on eBay)

If temps drop to closer to 20 or down into the teens, I have a few different options:

1. Add a lighter pair of wicking socks under the wool
2. Add a pair of neoprene socks over the wool (largest shoe size for this)
3. Add a pair of thin, windproof shoe covers between the shoes and neoprene booties

I ride down into the single digits for up to 30 minutes and any or a combination of those options work well. Never done a ride below zero yet... it don't get that cold here.

One other thing I'd point out.. the coldest part of my commute isn't the last mile, it's the third or fourth one. Feet and fingers sometimes start to get cold and uncomfortable, but then it gets better as my blood starts a pumpin'. Not that I'm suggesting you go out of your way to ride further just to find out, but one way to warm up is pedal harder!