Snow, lol.
Last week, we had about 8 inches fall within a couple of days. then it got cold, i'm talking -40C cold the one morning that I left for work. Last Friday it started snowing again but only for a little while, and environment canada issued a severe weather warning for the area over the weekend. Saturday afternoon it really started to snow, and Sunday morning I awoke to another foot on top of the 8 inches we already had. I headed of to work on my bike just like any other day. There was only myself and one other person that made it in that day...he drives a huge 4x4 and was totally shocked to see me pull in as even he had serious problems, lol.
Riding in the snow is similar to riding in loose, soft sand or mud. A decent mountain bike tire will get you thru it without much trouble. I've been riding year 'round for the past few years, and still haven't bothered with buying studded tires. Most bicycle shops sell them for around $30-40 a tire.
The nice thing about having an engine is that you do not throw yourself off balance by exerting weight from side to side as you peddle...this does help to keep you upright. The other nice thing is in slick, icy conditions you can also cruise with your feet down and use them like training skis (wheels) to help keep your balance.