Green Man Gaming offers downloads of Doom for Windows for $18.99 for VIP members. (Not a member? It's free to join; just signing into an account makes you a VIP.) Coupon code "BLACKFRIDAY10" cuts it to $17.09. Plus, get a mystery game for free with your purchase. That's $12 under our July mention, the best price we've seen, and a low by $3.