The first cold winds are blowing by my home; winter is coming to the northern hemisphere of our resource-drained planet. We're doubtless going to see a lot of questions about winter riding on the main site. Here are some sites that helped me learn how to ride in the winter.
This is one my favorite winter cycling sites. There's a lot of very, very good information here. The self-proclaimed "Home of the Winter Cyclist and Other Crazy People" is the granddaddy of winter riding sites. Started in 1998, this site is old: Beware moving and scrolling HTML.
This is cyclist Charlene Barach's page, and it has great advice about cold-weather cycling. The rest of her site is also well worth reading. Charlene has been riding in Canadian winters for years, and knows an awful lot about winter riding.
Pamela Blalock's site is quite comprehensive. She spends a lot of space on appropriate fabrics for winter riding.
In addition, I've learned a lot from some of the winter riding questions on this site:
I live in NYC and would like to ride to work during the winter as well. What should one do to a mountain bike? a road bike?
Note: This question has a lot of great answers, but AdamFranco's answer in particular is tremendous.
If I put on heavy clothes it will be very hot inside them after the warm-up. If I wear only a T-shirt I'll freeze. What's the solution? I ride around 20 km in a hilly city, with temperatures between 0 and 30 celsius.
I'm starting to plan ahead for winter riding, and gloves are definitely high on the list. What gloves work well for winter riding? Are cycling-specific gloves the way to go for a flat-bar bike with index/thumb shifters? Would regular winter gloves be good enough?
Disclaimer: I have the top-voted answer for this question, so my opinion of just how awesometastic this question is, is a little subjective. But I think that gloves are often overlooked when it comes to winter riding; it's hard to brake and shift when your hands are numb from the cold.