If you have Amazon Prime, you also have Twitch Prime included, where you get free PC games (similar to PS+ and GwG, but you can keep the games, even if the subscription ends).
Last week I got "The Banner Saga 1" and this week "The Banner Saga 2": https://blog.twitch.tv/twitch-prime-members-run-into-battle-in-the-the-banner-saga-2-for-free-1fee6dd7604d#.hp4s3qxp7
The only downside... just another game launcher. Now I have Steam, GOG Galaxy, Battlenet, Origin, Uplay, Win10 Store and the Twitch launcher for my PC games... and DOS-Box + ScummVM for my older PC games.