Green Man Gaming (second hand digital distribution site whose business plan is shakey is giving away a free copy of anything in their store.
Just enter '1FREE-PROMO-WORLD' at the checkout.
Details, including the best games:
Any PC game from Green Man Gaming – Free | SavyGamer
Shattered Horizons is also having a free to play half price weekend and Gog have a sale blah blah blah.