Space Engineers is currently featured in the Steam Free Weekend. The game will be available for free until Sunday, November 22, 1pm PST.
Space Engineers and Medieval Engineers will both be on sale for 50% off until Monday, November 23.
Get Space Engineers from here: http://store.steampowered.com/app/244850/
Get Medieval Engineers from here: http://store.steampowered.com/app/333950