Resistance 2 may look like Halo, but the experience is quite different and fresh in my opinion. I'm a bit addicted to it at the moment.
LBP is actually very fun online. You can play co-op with a friend offline, then switch to online anytime you want bringing a friend (or two) with you. Very hassle-free and smooth.
Pixeljunk Monsters is a great PSN game. It a tower defence strategy game (with co-op) that's easily given me over 100 hours of entertainment so far.
Uncharted obviously gets the vote as a great offline single-player shooting/adventure experience.