The excellent Theme Hospital the latest game to be free via Origin through their On The House promotions. Usually retailing at $4.99 you can get it for free right now and keep it forever.
Theme Hospital originally released in 1997 and looks dated but is immense fun. Design and manage your own hospital treating patients for things like Hairyitis, Spare Ribs and Slack Tongue.
To claim your copy, simply load Origin or head over to https://www.origin.com/store/free-games/on-the-house