Here's a cheap substitute for tire chains or studded tires -- use zip ties on your tires for extra traction on snowy roads. These will interfere with the brakes, of course, so it's suitable only if you have a fixed gear bike or some sort of hub or disk brakes. You'll also need to check for clearance.
I own a mountain bike and studded tires. Somebody let me know if this actually helps or not. The original instructions recommend placing these only on the rear tire, but experienced icebikers know that it's better to have traction on the front for control. I don't know, though, if zipties provide any control, or if they're just better for traction.