PSN - Warhawk is a definate buy, burnout is pretty good but expensive, pain is quite fun with friends as is ragdol kungfoo, Battlefield 1943 as said is very fun.
for a mindfuking experience, some games - Linger in shadows (Massive WTF, but definate try for only £1.50), Noby noby boy (trippy as hell), Flow, flower, astro tripper and super rub dub are very relaaxing and cool.
And All pixeljunk games are must haves, cheap with a hell of a lot of gameplay, monsters is very original and fun, eden is chilled and fun, shooter looks really fun as well.
Whats great about ps3 is that if you log in as someone else you can take any game they payed for, i have all these games and payed for only ragdol kungfoo and noby noby boy, its awesome