The list is broken out down by body region and lists both a transition season (when it begins to get cold, but before the deep freeze) and a cold season (below zero, Centigrade.)
Head
A helmet with a sun visor is typically helpful to keep rain drops off your eye protection or sunglasses when it's raining. It's also good for the months when you are lucky enough to ride in to the sun in the morning.Transition
A "Do Rag" is generally enough to wick the sweat off and hold in heat coming off your head. Remember, though, that if you lose too much heat through the top of your head, the rest of your body will be cold, too.
Cold
Cold weather brings out the balaclava and, in extreme weather, the helmet cover. Make sure that if you begin to commute more than once or twice a week that you have a couple of balaclava available since you'll want to wash them after about every use.
Hands
The trick with hands is to not have your gloves too tight. This restricts blood flow and allows for the cold to get to and stay on your hands. Cold hands make a bike ride miserable in winter.Transition
Insulated full finger gloves should be enough to get you through the transition period. If it's raining, a water and windproof outer layer complemented with a fleece base layer will be more than sufficient.
Cold
Cold weather brings out three layers of gloves. The fleece base layer mentioned above, an insulation layer to trap heat and a water and wind proof layer on the outside. If the mercury really plummets, a pair of skiing/snowboarding gloves can serve as the outer layer. They're bulky, but they keep the cold and wet out.
Feet
The only thing more miserable than cold, achy hands are frozen feet. The most important thing you can do to keep your feet warm is keeping them dry. If your commute is more than an hour or so, you'll also need to prepare to keep them warm even if it's dry.Transition
For the wet transition, the focus is on keeping water out of the inside of your shoes. We won't cover bike equipment here, but make sure that your rear fenders go all the way to the chainstay (to keep the water coming off the tire from ending up in your shoe) and put a mountain bike fender on your main stay near the bottom bracket to protect the shoes from the front tire. If your shoes are not waterproof, either buy some aftermarket waterproofing (not tested) or wear a light bootie when it's wet out. This will also allow you to strip off the mud before entering buildings.
Cold
When the temperature dives, it's important to have a set of shoes that do not constrict blood flow in your feet. It's also good to wear a second pair of wicking socks to keep any dampness off your skin. Booties will become a daily chore. While they are rotten to put on, they're an important part of keeping your feet warm.
Core
No matter the temperature, a wicking base layer is important to have on your core. In the summer, standard cycling jerseys work and allow for carrying of the essentials in the pockets. When it gets colder, you'll have a jacket on with pockets that's more convenient, so you can switch over to simple wicking workout shirts.Transition
A light, rain resistant jacket should get you through the transition. For those non-rainy days, simply a windproof jacket should do.
Cold
For cold weather, a wind and rain resistant jacket is critical. The jacket, gloves and balaclava should all come together to cover all skin, especially when below zero.
Legs
Transition
There will be days when, in the transition period, you wished you'd put a set of thin tights on. Any thin tight will do, but it's good to make sure the tight has a reflective surface as it'll soon be getting dark on your way home and your legs are about the only moving part on your body.
Cold
When it gets below zero, insulated tights are important, as extreme cold on the knees is very painful and leaves them injury prone. These tights should tuck in to the booties on your shoes.
