Games which you had to pay for that turned free.

Can't really think of one on top of my head, unless you count game bundles that came with a console (but that doesn't necessarily make the game free since it still costs money to buy the game separate from the console that bundled the game, and it isn't necessarily answering the topic statement). Usually games that are paid for that become free have issues of their own (bugs, lack of further development, etc.); if a game wasn't as successful, the developers might just make their game free simply for accessibility or to entice players who would not have played the game to try out the game. But I tend to play games that I know are successful, so I never run into a particular situation where the game I bought becomes free; most cases, the games I buy get discounted for a cheaper price, but not free.