So you're a bike commuter, and the weather is starting to get a bit crisp. You've never commuted through winter before for one of various reasons (no access to actual winter, first year commuting, winter usually drives you back into a cage, etc.) This winter, however, will be different. What do you need to be ready for?
The primary areas of concern are traction, braking, warmth, and visibility. I'll be giving you an overview of those areas, not a down-and-dirty how to. Your exact approach will depend on too many unknowns for me to tell you What To Do. Further research is highly recommended and almost certainly required.
Always take someone's background and location into consideration when you're listening to their advice! My last two winters were in Madison. Someone farther north in Wisconsin would need to be more prepared for bitter cold, whereas a cyclist in Oregon might need to be more prepared for cold rain as opposed to snow. Someone in a hillier area would be much more concerned about traction and braking. Figure out what's most challenging for you, and focus on that.
Traction is dependent on snowfall and what plowing and so on happens on your local roads. Traction will usually be compromised by either slick surfaces (ice) or soft surfaces (snow, slush.)
Studded tires will turn any flat, icy surface from an embarassment (on trails) or a deathtrap (in traffic) to a virtual non-issue. You should still, of course, approach unknown conditions with caution and moderate your speed.
The "studs" on studded tires are small metal nubs which protrude from the surface of the tire and dig into slick, firm surfaces to provide extra traction. Carbide studs, such as those found on Nokian tires, last the longest. Cheap tires will have softer studs that will wear away very quickly if used on pavement.
Fat tires will help in soft, loose conditions like snow or slush, where studs have nothing solid to dig in to. This is especially true when you run fat tires at a low pressure, so that they spread out and have more surface contact. Mountain bikes or some touring bikes will have the gobs of room you want for big, fat tires.
Everyone Loves an Extremist
The ultimate for snow would be a purpose-built Wildfire fat bike, Surly Pugsley, or similar custom snow bike. These will take specially designed tires that most bikes can't even come close to accommodating. The concept works the same on other loose, soft surfaces, for the beachgoers and astronauts among us, but may not be the best for mixed or unknown conditions, since they don't handle ice well.
Rutting? Get Fixed.
Icy ruts are the worst. The ruts will catch your tire and make steering unpredictable and difficult. If you're not slipping on ice, you're slugging through the soft parts. You'll want fat, studded tires, strong legs and steady arms for this. If you see this a lot, there's another traction control option which is well worth considering. Fixed gear bicycles are more often associated with bike messengers and the associated culture, but it's highly applicable to winter cycling. Particularly, the increased feel for and control over traction will keep you up and moving where you might have been involuntarily dismounted otherwise. Additionally, as long as you have rear wheel traction, you can resist the pedal motion for braking.
Refraining from Moving Forward
Once you've got traction, you can start moving forward. This is an excellent thing, especially if you and your boss enjoy it when you get to work on time. On your way to work, however, you may encounter such obstacles as stop signs, dump trucks with right of way, moose, etc. At these and various similar junctures, it would be wise to refrain from moving forward. In fairly mild conditions, you may find that your current brakes are just fine. However, if you mix rim brakes and harsh conditions, you may run into problems if the rims ice up, and when I say problems, I mean like dying.
If you're looking for something more reliable, there are a few options. As above, a fixed gear will let you apply resistance through the pedals to the rear wheel. As long as the rear wheel maintains traction, you can brake. Disc brakes may be more resistant to icing up, due to the braking surface being higher off the ground at all times. Internal hub brakes (drum brakes, coaster brakes, etc) may be the most reliable, provided the hub is well sealed, though they're generally not as powerful. Still, in winter, you need reliable brakes far more than you need powerful ones. If all else fails, try to aim for a big pile of snow which does not contain any fixed objects.
One moderately popular winter activity is not quite freezing to death. I am personally a huge fan of wool clothing, and I highly recommend it for winter biking. Cheap wool sweaters from your local thrift store are a good start. Whatever you choose, the idea is to maintain a balance - if you're too warm, you'll start sweating, and being wet when the wind chill is -30F is not a survival trait. It's a good sign if, when you walk out, you're a little cool but not cold. Layers are good. Be prepared to remove and carry extra layers, or add more, as needed. You can also moderate your level of activity - go faster and you'll get warmer, take it easier and you'll cool off.
Visibility is always a concern, but especially in winter, with shorter days and nastier weather. You'll want a bright headlight and tailight, and maybe more than one of each. Consider generator lights - contrary to what many people seem to think, current generator hubs are very efficient (the extra effort for most decent generator hubs is equivalent to climbing a few feet per mile.) It's a bit of an investment, but often worth it to avoid having to worry about batteries dying. You can also augment your active lighting with passive lighting, like a reflective vest, reflective ankle/arm bands, and so on.
Your ability to see should also be defended. If you're out during active snowfall, bitter cold, cold and windy weather, or bright sun plus lots of white snow, ski goggles can be very helpful. They will help keep you warm - combined with a balaclava, you can nearly eliminate any exposed skin. They're generally tinted to help deal with snow glare, and they'll keep wind and snow out of your eyes. I wear eyeglasses already but an eyeglass-friendly pair of ski goggles is still very, very useful in several conditions. If it doesn't very cold where you live, they may be overkill.
Try to stay in the saddle as much as possible. Standing up moves weight away from the rear wheel. Weight is traction. Traction is Life.
Try to leave your bike sheltered but not warm all of the time. If you bring the bike into a warm house, condensation may draw water into the brake lines, etc, where it may freeze when the bike goes back out. Avoid the problem by never bringing the bike into a warm place, if you can.
Winter is very harsh on your drivetrain, especially if the roads in your area are salted. Keep your chain lubricated and clean out the muck as best you can. As a year-round lubrication solution, consider paraffin wax. Keep a crock pot of paraffin handy, start it warming, and clean your chain. Drop the chain in the melted wax and let it sit for a bit. Hook it out with a spoke, coat hanger, etc, and let it drip. The wax doesn't pick up dirt as easily as most oils, but it also said to not last very well in wet weather.
As a long-term planning goal for bike commuting in any weather, try to arrange your workplace, home, and schedule such that morning commutes are westbound and afternoon commutes are eastbound.
The ICEBIKE webpage and list are good resources, though you cannot access the ICEBIKE list archives without being subscribed. Traffic isn't too bad and it's a low-noise list. The bikelist.org lists are also worth checking on for general cycling talk which does tend towards winter stuff during the season. Those archives are searchable from the website.
For bike parts and service, try to find a decent local shop first - but keep in mind not all local shops are decent, and even ones that are may have no interest or experience in winter cycling. For lighting and studded tires, Peter Jon White's website has an excellent selection and good advice, and he is in many cases also the importer for the products he sells. Harris Cyclery is a good option for online ordering of general parts.
For clothing, Icebreaker and Ibex sell excellent, super-soft wool products. Your local thrift store, again, can be an excellent resource, especially for all-wool sweaters. Sierra Trading Post has decent deals sometimes, especially for things like last year's ski goggles or wool socks, etc.
Winter biking poses a number of new challenges for the commuter which must be solved for comfort and safety to be maintained. With appropriate research, planning, and preparedness, however, you can extend your commuting season into almost any conditions. Take things at your own pace, try short test rides if your commute is long, and learn your lessons before things get too cold.