For those hearty souls that keep riding regardless of the weather, specialized tires for riding on snow and ice can help maintain traction under trecherous conditions. Studded snow tires are available commercially, like the ones shown above, or can be made by do-it-yourselfers by running small machine screws through a tire from the inside out.
In both cases you'll want to use these tires only when there is ice and snow present. Running around with them on dry pavement will either wear down the studs or damage the tire or both.
Just as a side note, one McGyver-type I heard of used plastic zip-ties around his rims and tires to give extra traction. That's pretty much a single-use solution however, as you can't then remove your tire again until you cut off the zip ties. Plus, another minor consideration is that if you have caliper brakes that slow you by squeezing brake pads against your rims, this zip-tie solution really isn't an option at all.