Ski goggles. I got them one year when the windchill was strongly in the negatives. Now my glasses don't fog up, my eyes don't hurt from the cold, and I don't get glare-blinded by the sun. Best $40 I ever spent. Probably more like $50+ in today's money.
In a similar vein, I got some high quality mittens this past year and they've been incredible. I moved from California to Colorado recently and my previous finger gloves just couldn't hold up to the Colorado cold.