Yeah I'm with the popular consensus on this one, the MUST HAVES in my book though.
Disc:
Uncharted.
Heavenly Sword.
Ratchet and Clank.
Warhawk.
DMCIV
Rock Band
Burnout Paradise.
GT5P
Condemned 2 (I know I know first person shooter, but man it's good :D )
PSN:
Super Stardust HD
Pixel Junk Monsters.
Everyday Shooter
Pain (if you have a few people willing to play it, it CAN get old fast :P )
Also note, make sure you pick up the Echochrome demo, and pick up Castlevania SOTN for PS1 classics :D
From 0 to KICKASS in .stupid seconds.