The BTABlog reminds us that if you're riding your bike at night, lights are important! The police used to distribute lights, but when it comes down to it - riding a bike is so much cheaper than owning a car - it is worth some small gear investments.
We at Worldchanging/Portland recommend lights, a helmet, some reflective tape, a bell, and some good rain gear. Additional bonuses are a rack and a pannier, which make carrying larger items simple.
If you aren't up for assembling all the gear yourself, Clever Cycles has some amazing bikes which incorporate many of these features into the bike itself.
Remember - the rain is no reason to stop riding. All you need is the right gear and you're good to go. I actually prefer winter riding because the trails and bridges are so empty. I suppose if everyone follows my advice, this will cease to be the case, but for now I'm not worried!