Outrun Online Arcade - Fun arcade racer
Soldner-X - Rock-hard side-scroller shooter
Braid - A brilliant puzzle platformer (I don't know how much it is on PSN though, but if it's no more than £8, go for it)
Bionic Commando Rearmed - Short game but really really hard, great remake of a 20-year old classic
To name a few.
The soundtrack is excellent and the gameplay is extremely addictive.