It's raining pretty heavily right now. Some people think that foul weather prevents cycling completely, but this is not so. I cycle-commuted in Detroit for a number of years, and I've learned how to do so comfortably in any season.
The most important accessory for foul weather cycling is fenders. "Oh, gawd, no, not fenders! How dorky can you get??" I hear the cries already. But the foul weather cyclist has only two options: put fenders on the bike, or get covered with road fly. Filthy water from the road will cover your back, legs, and face if you don't have fenders. Granted, I only put fenders on my fixed-frame ATB (rear suspensions make it very tough to mount fenders) and not my road bike. Zefal's plastic fenders are pretty good.
The next most important thing is Gor-Tex. Gor-Tex is completely waterproof, completely windproof, oil-repellent, and it breathes -- meaning it allows your sweat to evaporate (somewhat). I can't tell you how much difference it makes to have Gor-Tex socks, pants, and a jacket.
Get the socks first. There's nothing like a pair of Rocky Gor-Sox to keep your feet dry and warm, and they're the least expensive piece of Gor-Tex you'll own, and they'll make the biggest difference. No fenders will protect your feet from road fly, but Rocky Gor-Sox will. Not even neoprene (wetsuit material) booties will do so good a job.
Get the pants second. For one thing, they're cheaper than the jacket. And odds are, you have a lot more layering options for your torso than your legs. I'd guess that my usual commute pace on my fixed-frame ATB was about 17 mph, and Gor-Tex pants kept my legs warm enough that I only wore shorts underneath in freezing temperatures. If you can't find a shop that carries them, try Bike Nashbar, Performance Bike, and REI. Velcro ankle closures and ankle zippers are both very useful. If they weren't cut specifically for cycling, get them a couple of inches long.
Last comes the jacket. It's probably best to get one made for cycling, as there will be many useful features not found on other gor-tex jackets, such as an extended tail, extra-long sleeves, reflexite, and vents in the underarm and back. Again, in subfreezing temperatures, I wore only a basic shirt under my gor-tex jacket.
Winter layering, for me, consisted of light fleece gloves, 100gm Thinsulate gloves, a fleece headband, and a fleece neck gaiter. At the last resort, I'd tape over the front vents of my helmet. (When cycling, I ALWAYS ALWAYS ALWAYS wear a helmet! They're so much nicer than brain trauma...) The neck gaiter is so much better than a scarf -- it never unwound, and I could pull it up to my lower lip or pull it down below my chin, at need. With just those over shorts and a T-shirt, I was comfy riding in 20 degree F (-7 C or so). With sweats or polypropylene underneath the Gor-Tex, I expect I could have handled even the infamous Winter of '93, with its constant clear, cold weather and -40 windchills.
If gor-tex outerwear is too costly, consider some of the other windproof, water-resistant, breathable shells. They are far less expensive. This works because the main reason a cyclist gets cold in the winter is wind chill, not snow. Snow usually brushes or blows off most clothing. With fenders, the only place where complete waterproofness (and thus, Gor-Tex) is vital is your feet. A little bit of layering, particularly with some high-wicking stuff like polypropylene, will help deal with whatever water gets through such shells.