Plus+ is a service similar to Xbox Live, but it's 100% free, and using push notifications on the iPod Touch/iPhone. So far it displays what could be included in many games and apps to come, but for now it's been exclusive to ngmoco games. Those games include Star Defender only for now, but games liek Kill Test, Rolando 1 and 2, Dr Awesome and others will be included. You make an account within an app too, so you do not have another app clogging your screen.
Official plus+ site
Official plus+ site