Popular subway line building game Mini Metro is free in the iOS App Store for a limited time.
The game has won and been nominated for several awards, plus it currently sits with an impressive 4.9 rating in the App Store.
The goal of the game is to construct an efficient transit system. It was originally released in various alpha and beta versions from 2013 to 2015, before officially making its way to Mac and Windows. It released on mobile the following year on iOS and Android.
So far, Mini Metro is only free on iOS.