The first piece of gear I'd recommend is a bus pass. Taking public transportation is safer and warmer than chugging to work between traffic on a slippery, slush-clogged road. But that's me. If you're psycho enough to do it, and you live in a cold-weather clime, your first investment should be in reflective and lighting gear--because odds are, you'll be riding home in the dark for most of the winter. Sigma Sport makes an extremely battery-efficient LED headlight for commuters called the Triled. The company also offers a five-bulb LED tail light called the Tailguard. And don't forget a dorky vest, in case some driver is too busy sending a text message to be able to spot your lights. Nathan Sports makes a trusty, no-frills one called the Cyclist's Vest, which sells for $22.
Next comes clothing. The biggest mistake is leaving skin exposed in cold weather--just like if you're skiing. Gore Bike Wear, the in-house apparel label by the makers of GORE-TEX, is your one-stop shop, here. Its Universal Hood ($50) is windproof, waterproof, and adjustable, and its fleece-lined Radiator gloves ($60) are gel-padded at the palm and highly wind-resistant--though the four-fingered design does make your hands look like they belong to Homer Simpson.
The go-to jacket for commuters is the quilted, full-zip and aptly named Pearl Izumi Insulatour ($150). It's aerodynamic and warm, yet its polyester fill and shell don't trap in the steam. For pants, go for a standard waterproof and breathable pair, like the dependable REI-brand's Novara Express Bike Pants ($47).
And if you're really hardcore, and will go out in weather that keeps even the Postal Service home, consider studded bike tires like the Schwalbe Ice Spiker ($110).