levelTradingField.com has a prediction market for Bitcoin. You basically predict what the price of Bitcoin (using the BRTI index) will be at the end of the hour.
the way it works is you are put in a group of 10 people. Each person makes a prediction and puts a fixed amount of money ($0, $5,$10,$20,$50,$100) in a pot.
3 people win as follows: The person with the most accurate prediction at the end of the hours get 50% of the pot, the next person gets 30% and the 3rd most accurate gets 20%.
the site has a Bitcoin statistics application to help with predictions