I was staying right downtown, so the bicycle messengers (like the guy on the right) were in view practicing their trade. Their favorite _weapon_ is the 'fixie', a very old-fashioned kind of bicycle that not only does not have 27 or 30 gears like is starting to become common on the bicycles of today, but only has one! And this kind of bike cannot 'coast' (because of a direct link between the pedals and the rear wheel), so if it's moving so are your feet around on the pedals even if you are going downhill.
It is this very last feature that makes the fixie a favorite of the serious winter rider -- the claim is that the direct link to the rear wheel gives you excellent _feedback_ about when you're starting to lose traction on a slippery surface.
Here is one fixie I saw:
And here are two parked next to one another:
Here's one guy making a fast turn under the "EL" (apologies for the finger in part of the pic . . . and for the general low quality of the images; I was just using my camera phone):
And, finally, here's a guy coming back out to his bike after making a delivery:
Hardy souls of Chicago, my 'hat's off' to you!!!! Long may you ride.