PSN plus is the premium service you can opt to pay for on the PS3. This is only an add on and online games are still free to play with friends.
The perks of being a subscriber include.
- Free games every month that you own as long as you still subscribe
- Full game trials where they give you an hour to demo a game
- Exclusive games and early acsess to beta’s.
- Automatic Download of updates
- Exclusive PSN discounts
You can only subscribe in chunks of 3 to 12 months at this time.
3 months will cost 17.99 US and 12 months will cost 49.99 US.
There is also a promotion for Europe where if you subscribe between the 29th of June and the 3rd of August you will receive a downloadable copy of Little Big Planet that is not tied to you subscription.
(Source: EU Playstation Blog)
(Source: NA Playstation Blog)