Super Stardust HD is the classic recommendation for PSN games.
While only local multiplayer, hands-down best dual-stick shmup out there on any platform. If you like shmup, it is basically a mandatory purchase.
PixelJunk Shooter, Eden, and Monsters are fantastic as well. I know Shooter and Eden has some fun co-op multiplayer. The visual style and excellent music coupled with simple but solid gameplay might appeal to more casual gamers as well.
Fat Princess is a lot of fun. It is like a TF2-style Capture the Princess game in third-person perspective (like an RTS). Unfortunately (or fortunately?), PS3 owners rarely have mics and use them, thus the team-focused game tends to suffer for that.
Marvel vs. Capcom 2 is simply fun, like it always has been.
For other small titles, I like Bionic Commando: Rearmed and Shatter. You can also get those on Steam though.
For full-on retail-quality game, you have Wipeout HD with Fury Add-On Pack. Kind of a hardcore futuristic racer. It is also probably the most visually impressive racer until GT5 comes out. I like it a lot, but some ToJ'ers don't find it that appealing.
But yeah, Super Stardust HD would always be my #1 recommendation for a new owner's first PSN title (unless they don't like shmups).