Green Man Gaming takes an extra 12% off a selection of game downloads for Windows, Mac, and Linux via coupon code "GMG12" during its Holiday Sale. A couple of best bets, with prices after coupon:

Ori and the Blind Forest: Definitive Edition for PC for $7.92 (low by $2)

Battletech for PC/Mac/Linux for $19 (pictured, low by $4)