I can see it now - you sign up and log where/when you're going, and how much space you have to take packages/letters with you. Some algorithm matches your as a courier up with people who need stuff taken where you're going.
The service is free if you act as the courier (...or mule, I can totally see that being misused rapidly).
I don't know how you get around the depot and 'guy who goes door to door' model though.
In a way, Fedex/UPS already do this...