Pirate to Paid Participant
As a rule am trying to pick out games I had very low interest in before pirating them but afterwards my interest was significant. As close to "Piracy sold me on this game" as I can get, though of course some of these I might have bought eventually anyways, purchasing them was done well before that because of piracy.
List of games where any/all pre-release material I consumed did nothing to sell me but after actually playing the game via piracy I felt compelled to own it, even if I'd actually finished playing it. Nothing says you love a game more than spending money on it when you've already experienced it, and that first experience costed you nothing.
It's hard to remember past games fitting this description so I'm sure there will be a few that I've missed that I may fill in as I remember.