You can download demos via PS Store. PJ Monsters is pretty good, Super Star Dust was very fun while it lasted.. very hairpulling game so to speak lol. Stay away from Pain.
Also, I have Brain Challenge, it's lots of fun if you play with friends at your house. I have downloaded alot of PSN network games, nothing really just shines out IMO. I just like doing the Trophies in the games and such