In light of the fire at Notre Dame, Ubisoft are giving away Assassin's Creed Unity, which you can pick up for free here. You need an Ubisoft Uplay account and the Uplay software.
This is actually the only Assassins Creed game I've ever wanted to play specifically because it has what looked like an amazing reproduction of the cathedral in it, so I might even brave installing Ubisoft software on my PC again for this one (last time I did this I got caught up in their DRM debacle which repeatedly crashed my PC so I'm wary).
Uplay is much improved from the bad old days, to the point that I prefer it to Steam. Unity itself is meh, but seeing Paris as recreated in the game is fantastic and they patched it out of the terrible state it launched in.