For Portland winter -
Fenders, serious headlight (dual halogens for me), blinking tailight, plus headlight and blinking tailight on the helmet. Specialized Armadillo tires, don't want to be fixing a flat in the cold and rain.
Waterproof pannier - I use this one, expensive and hard to find but works great and can be used as a semi-briefcase.
http://www.ortliebusa.com/cartgenie/prodInfo.asp?pid=106&cid=2
Rain jacket (yellow Burley), rain pants, medium fleece, bike shoes, full-finger gloves. Chilly for the first few minutes, before the body warms up. I get to work dry enough to not need a change of clothes, though I keep a change in the office just in case - though I don't need to impress anyone with the crispness of my pant creases either.
I've often thought a small fairing would be a nice thing too.