There aren't really any verified sites that has a list of games that gives out free vouchers and free items. Although there are a lot of applications that gives out the same thing that you are looking for. Exp game is one of the applications that are verified to give out free vouchers. They Have I love Pasta, Demons Hunter, Game Of dice and Conquer age if I remember them correctly.
For Diablo III, they don't really give out free items unless you join events that are made by admins. They usually held those in Facebook groups that are for those games, although that isn't very sure. Same as Cabal 2, Game masters are entitled to give free vouchers when you join their event.