Little Mom on the Prarie made game tokens that she gives her daughter to do chores.
"I started using an alternate version of the reward system with my daughter, to balance out our struggles of getting her to help out with chores and limiting her computer/game time. I made these little "game tokens" that she can earn by doing her chores without complaining, and when we ask her to do them (not later, not in 5 minutes, now!) Each token has a different time allotment on it; 15 minutes, 30 minutes, and there is one token good for 1 hour.
It would be fun to add RFID chips to the coins so that the kid can spend the coin by passing it over a USB-RFID reader connected to the computer.