Front and Rear Fenders– Essential for preventing the wet pavement from splashing all over your clothes.
Riding Gloves– Not just any gloves, but riding gloves. They keep your hands warm, but are also breathable so moisture/sweat doesn’t get trapped inside.
Waterproof shell jacket w/ ventilation: Combine a shell jacket and a couple of layers (depending on how cold it is), and you have yourself a formidable pair to combat any type of inclement weather.
Waterproof Bags– Messenger bags, backpacks, pouches, and hip packs. The only way to keep the water out of your precious belongings is to have bag that is waterproof.
Scarf/Neck gator– Exposing your face and neck to the harsh winter winds can make riding a chore, rather than a fun activity.
Headgear– I think the saying goes that you lose 90% of your body heat through your noggin. Add a cycling cap or beanie underneath your helmet for added warmth.
Arm/leg warmers– Not only do these keep you warm, but they also wick away sweat for a much more comfortable ride.
Base layers– Wearing cotton t-shirts underneath all of your layers can add to the sauna effect, so investing in a nice polyester type base layer will keep you warm and dry.
You can pick most of these items up at iMinusD.
Here Matt is ready for the rain with the Mash Histogram Cycling Cap, Continental Rain Jacket, Specialized Edge Gloves, Osloh Track Jeans, and is running the front and rear SKS Fenders.