PS3 Local Multiplayer

Castle Crashers :)

if you go resident evil go 6, 5 has its share of not being that great

if you want to local multiplayer, always go for older consoles.
this whole inter connected drm concept is b#s#. I just came out of psn because I couldn't stand for lack of everything. :}

Blur

Depends on your girlfriends taste. You can never go wrong with LEGO titles.

also diablo at late game will lag the ps3 if you play together, escipecally witch doctors.

These are the best imho:

Diablo 3
Resident Evil 5 & 6
Army of Two (series)
Borderlands (series)
Lord of the Rings: War in the North
Little Big Planet (series)
Eternal Sonata

It's depends on what you like, personally I loved:

Metal Gear Solid Collection (Play this now if you haven't played them!)
Tokyo Jungle
3D Dot Game Heroes
Folklore
Ni No Kuni
Infamous 2
Uncharted 1
The Last of Us (Multiplayer)
Little Big Planet 2
Persona 4 Arena
The ICO & Shadow of the Colossus Collection
God of War 3

Try Sacred 2, i believe that has split screen and its a little like diablo but more quest and story i think. Also try looking here http://www.co-optimus.com/system/2/playstation-3.html, it should pretty much list every ps3 game that has local multiplayer.

Guacamelee!

Modnation
Dragon's Corwn

Co Optimus is a great site to find co op games. It has a local filter too. Below are my personal recommendations:

Fighting games generally all have local mp. Notable ones that come to mind are Tekken Tag 2, Ultra Street Fighter 4, Mortal Kombat, Playstation All-Stars Battle Royale, and King of Fighters.

Not all racing games have local mp. I'm pretty bad at racing games in general but I must recommend the Motor Storm series (namely MS: Pacific Rift and Apocalypse). Motor Storm has split screen multiplayer, easy to control, and fun to ram your friends into a tree/wall/volcanic lava/etc.

Music games are great at parties as you can have up to 4 people jamming along. Singstar, Guitar Hero, Band Hero and Rock Band series are all great but they do require the peripherals which should be pretty cheap now.

Little Big Planet series is a sandboxy game like terraria. PSN has plenty of user created content (similar to steam workshop)