Two of the best PSOne Classics are being thrown in as well on May 1: Castlevania: Symphony of the Night and Castlevania Chronicles, a remake of the original 'Vania. Saints Row: The Third ... isn't free, but it is 20% off for Plus users when it comes to PSN May 15.
For more, including free Minis and discounts on more upcoming PSN games, check the PlayStation Blog. For the answer to the tough question of whether relatively cheap access to all these games is worth an ongoing subscription, when you know you'll lose them when your subscription lapses, check your heart.
Sony PlayStation 3 (late 2012)
Sony PlayStation Portable PSP-2000