Winter isn't the kindest to runners. The cold air burns our lungs, the snow soaks our sneakers, and the ice can cause some nasty falls or injuries. There's always the treadmill, but it's so boring, and sometimes, I just feel like a hamster in a wheel.
If you have access to a pool, here's an option — run in the water. You can try treading water where you can't touch the bottom, since it's great for your cardiovascular system and your muscles. The only problem is it can get tiring quickly, and you'll only be able to tread for a few minutes. In order to be able to do a longer "run," I recommend getting a flotation belt ($25). They're fairly inexpensive, easy to strap on, and they won't get in the way of your arms or legs when you move.
The water offers amazing resistance to your entire body below your head, so running underwater is actually much more challenging than running outside. It's also easier on your joints, since you're not jarring your feet with each step. The water also keeps you cool, so you're not at risk for overheating. If you don't have a flotation belt, or don't have access to deep water, you can just walk or jog on the bottom of the pool, and still reap the benefits to your muscles, heart, and lungs.