I'm a massive fan of anything I can use my iPhone to earn money from. I stumble across Qustodian a few months ago and I have to say that I've not been disappointed. In simple terms, you earn money for every message you read through the app. You're alerted to new messages once a day if you have push notifications turned on.
Now of course they don't market it as a money making, but rather as a service that send you personalised and targeted adverts. It just so happens that I'm interested in every category they have listed, so I open more messages. You don't even have to click through to earn anything, just read the message.
It is a slow earner – payout is at £10 into your Paypal account. I reckon I'll probably get a massive two cash-outs per year, but the app really isn't intrusive and the messages are not time sensitive, meaning you can wait until you've accrued a few before opening them.
ETA January 2014. I'm still very impressed with Qustodian – I have been paid twice and I'm already up to £9.72 again.
ETA: July 2014: Well, Qustodian are no longer paying out as much – read more here.