In any case, this topic was at top of mind yesterday, since it poured all day long, and I was out on the Christiania with the girls. I took them to school, then rode to Alewife, then the CC office, then pickup and home - all in the most torrential rain.
For the ride, I wore my nice bikey 3/4-length shorts, Smartwool mid-layer long-sleeve shirt, and Patagonia base layer. This was all moderately presentable in a bikey sort of way so I wouldn't attract too much attention on the T or at the office. Nice and comfortable too, with Andiamo bike underwear. None of that is waterproof, of course. So on top, I wore Gore-tex overtrousers and my Gore-tex cycling jacket. Both by Freestyle, a now-defunct British company. (Which is a shame - I love their stuff, and the customer service was great.)
Those are good basics, and they keep me dry in most drizzly conditions. Yesterday I was wet round the neck and forearms because it was raining so hard. I brought my Tevas to change into but rode in LL Bean hunting boots, which are wonderful as long as I tuck my overtrousers over the tops to prevent them filling with water.
My head was kept reasonably dry thanks to a Gore-tex helmet cover, and I wore a cycling cap underneath so that the bill would keep water out of my eyes.
When it gets really cold, I stick to the same sort of arrangement with a couple crucial additions. I wear nice wool socks and my hiking boots, which keep my feet warm and dry. I wear balaclava and sometimes a scarf depending on how cold it is, and a nice wool cap. I also wear a long-sleeve wool jersey or mid-weight fleece under the Gore-tex jacket. On the bottom, I have Endura winter tights, and depending on the temperature, I often wear long wool tights over them. And if it's wet, I'll also do the overtrousers. Finally, I have some heavy Pearl Izumi gloves that I wear on their own or with lightweight wool liners.
If you open up a high-end bike catalog these days, there are all sorts of Logan's Run-inspired outfits that can easily cost more than $500. In the winter, I'm not trying to set any speed records, so I'm usually happy with my layering approach. It only gets a little tricky when I strip off and have to find a place for all my stuff to dry.
(The most important point here that may have been lost among my rambling is that the best stuff is either Gore-tex for waterproofness or good quality wool for breathability and warmth. My fave wool stuff is from Kucharik (hats, gloves, tights) and Earth Wind and Rider (jerseys - made right here in Boston!))