Killzone 2 is an amazing FPS experience. The only problem is that the aiming controls are all fraked up. I have a Fragnstein PS3 mouse controller, but on Pad it is hard. Maybe try first. If you can handle the aiming... it's one hell of a ride and is my favourite PS3 game.
Uncharted and Uncharted 2. The second one really stands out and is much better, but if you want a complete experience in the series the first isn't too expensive.
LittleBigPlanet is a wonder. It's especially awesome with multiplayer. Make your friends play with you. Even if single player only, it's a treat. 2D platforming fun.
Valkyria Chronicles is a fun strategy RPG. Not everyone's cup of tea, but I like it. Anime style, but if you can muster that it's pretty good.
Ninja Gaiden Sigma. Only if you didn't play Black on the Xbox 1 because it's basically the same. I like this series way better than Devil May Cry, and even a bit better than God of War (Gameplay wise - they cannot possibly match GoW's epicness).
Then there's Ninja Gaiden Sigma 2 which is a revision of the 360 NG2, more than Sigma was it really is a lot different (easier, but new characters, bosses). I've always been disappointed in the lack of Ninja Gaiden interest when they are so much better than DMC.
Flower.
FLOWER.
WHY DO YOU NOT HAVE FLOWER YET????
Flower is possibly the best PSN game. You play as a flower petal and collect more by controlling Six Axis. It's $10. It's relaxing, fun, pretty graphics. Kinda short, but it's a unique experience.
Echochrome is a unique PSN puzzle game based on perspective. It's a puzzle game in the same way that Portal, Braid, or World of Goo are. By this I mean not Tetris, but an alternative game that can only be described as a puzzle. What you see is what you get in this game (perspective alteration), and for $10 it's the other must have (alongside Flower).
Shatter is another nice PSN game. As an Arkanoid fan I enjoyed it. At $8 it's a hit on the wallet small enough to be worth taking.