Mass Effect is a good series, but as a PS3 owner, you'll only be able to get ME2 and ME3. Choices from the first game are made by you in a sort of special checklist thing upon starting ME2, or so I've heard.
Assassin's Creed is an outstanding series, especially from the second game onwards. Lots of good old fasioned city scaling free roaming platforming stuff, plus the ocassional assassination.
Sandbox-wise, you've got Grand Theft Auto IV and Red Dead Redemption, great, solid games.
Silent Hill: Downpour just came out, and should be good, if your into some horror.
The two recent Batman games (Asylum and City) are also pretty solid, too.
There's also Uncharted, if you feel like knocking down a few ancient temples.
Oh, and Just Cause 2. Gotta go with the crazy open world revolution stuff. Defenitly worth your time.
Skyrim, Skyrim, Skyrim, if you want a time sink.
Finally, if you have one of the original PS3s, (and I mean original, release day consoles for the PS3 line) you should be able to play PS1 and PS2 games, so you can dig around there.
Just thowing games sat the wall, see what sticks. It's science, you know. Also, that reminds me, The Orange Box, Portal 2, and if you have the original PS3, Half Life 1 (PS2).
Happy gaming!