GOG cuts up to 80% off a selection of game downloads and DLC for Windows, Mac, and Linux as part of its Best of Summer Sale, with prices starting from $1.49. Deal ends August 1. A couple of best bets:

Rebel Galaxy for PC and Mac for $9.99 (low by $10)

Soma for PC, Mac, or Linux for $17.99 (pictured, low by $12)