Free Game a Day gives away free PC games AND Mac games. Usually Steam keys, but also Origin, Battlenet and Rockstar keys.
Every first of the month they will draw a winner at random, and post the winners on their social channels with screenshots to prove it. So keep an eye on their socials and don’t miss anything.
Why do they give free games away do you think? Short answer: This is their way of contributing, and they LOVE Games!