A new set of studded snow tires.
Ok, so I've put off buying snow tires for years--maybe a decade. I'm not sure why, maybe the price. And maybe I didn't think they'd make that big of difference. Well, I purchased this set today and I cannot tell you how incredible they are. The downside is that they greatly (really greatly) increase road resistance, which is especially noticeable on dry pavement. But man, in the snow I feel like I'm driving a four wheel drive truck. No slipping or sliding; excellent braking. The money spent is well worth it. And if you happen to be a four season cyclist who is considering purchasing these tire but haven't yet, just do it. You won't be sorry.