I finished inFamous a couple of days ago and it's awesome. The whole open game world is so overused these days but inFamous actually feels new and fresh. I loved Cole's powers, I loved the story and I loved making good or bad choices. And the ending actually surprised me.  My Cole was a real boy scout who helped everyone but I can't wait to play through the game again as a bad dude. Gimme those red lightnings! But if I'm completely honest I don't want to play it again just yet.  
 

I finished inFamous a couple of days ago and it's awesome. The whole open game world is so overused these days but inFamous actually feels new and fresh. I loved Cole's powers, I loved the story and I loved making good or bad choices. And the ending actually surprised me.  My Cole was a real boy scout who helped everyone but I can't wait to play through the game again as a bad dude. Gimme those red lightnings! But if I'm completely honest I don't want to play it again just yet.  
 

Not sure if you're into racing, but ModNation Racers offers a ton of gameplay and the customization is really endless.  Might be worth a look, unless you've already played it.

Don't play Prototype.  :| 
 
Check out Red Dead.

Well those games you mentioned are the best the PS3 has to offer. 
 
So probably just pick up some mulitplatform games.

Valkyria Chronicles. Uncharted.

Ratchet and Clank  
or you could watch MGS4 on blu-ray.

"  or you could watch MGS4 on blu-ray. "

You're probably not a Metal Gear Solid fan, or you would have already played MGS4.
 
If you haven't, you should definitely play through the entire series. Otherwise, Demon's Souls will last you a good 80 hours or so.

Trust me, stay away from resistance 2. It's not good at all for single player. I liked the first game though even if the gameplay is completely outdated. The second game, well it's like you die if you don't stand where ted price wants you to stand and that's on normal difficulty. I died more times than all other fps games i've ever played combined. Totally bad gameplay decision, guessing where to stand and shoot and trial and error.  
 
For recommendations, I don't know, maybe you feel like some of the games are a bit overated and you aren't feeling it. I understand, I feel the same way too and absolutely think infamous is great. Mgs4 has alot of cutscenes, I wouldn't recommend that unless you are a mgs fanboy or you won't understand it. It's practically an interactive blu ray movie. God of war 3 might be okay, however it's not really fun, not sure why though. Infamous would of been my recommendation, I don't know what to recommend because killzone 2 was my next recommendation you already played.

God of War III.

Some people have a problem with it but if you can get into Demon's Souls it is a great and unique game. I have also heard good things about Valkyria Chronicles but have yet to play it myself.

You should get Valkyria Chronicles and Demon's Souls.

not naming games people already said 
 
Noby Noby Boy 
Castlevania was made for PS3 pretty much
3D dot Game Heroes 
Disgaea 3 
Katamari Forever 
Mag 
Mod Nation 
Motorstorm Pacific Rift 
Ninja Gaiden Sigma Series 
Siren was received well
Yakuza

Get the God of War Collection, play through the first two games, then get God of War III.
 
Also, there's no reason not to play Uncharted: Drake's Fortune.

Metal Gear Solid 4?

God of War 3
Castlevania Lords of Shadow
Yakuza 3
Ratchet and Clank - A Crack in Time
MGS4 (if you havent played it)
The Saboteur
 
All are very good games.

Why thank you, I was and still am awesome.

...the PS3's biggest exclusive is out in two days.  You could try that?

I did forget that GT5 will be out in a couple of days... should be well worth it.

I'm currently playing Uncharted: Drake's Fortune and it's great. Not as good as Uncharted 2 but still one of the more enjoyable games on PS3. Next on the line is probably God of War 1 which I have already started and played for a few hours. 
 
GT5 doesn't really interest me that much. Forza 3 and F1 2010 on Xbox 360 satisfy my sim-ish racing needs.  

If you didn't play em on PS2, Sly Collection maybe? It was made by the same folks.