This Java ME multi-player, location-based game in South Africa, called "Legends of Echo", lets you play for free mobile airtime, or (for everyone's favorite) cold. hard. cash.
See:
Play for airtime or cash
Here's a quote: The game is a free Java download for cellphones, and once players have it they can immediately start playing. There is no cost to play, except for the standard data costs to the network provider. Players can trade airtime for Elements, which can then be traded for weapons. That's the nice part of Java ME technology: Developers push to innovate new types of games, especially location-based games that are multi-player. Look to Java ME developers to lead the others in new ways to make money on mobile phones.
Hinkmond

Read More about [Mobile game lets you win cash or airtime w/Java ME tech...