How to use block and transaction hashes for generating simple stochastic chances?
Here's the way SatoshiDICE does it:
SatoshiDICE has "secret keys" for each day which they pre generated for the next 10 years. They published the hashes of those keys so you can verify they didn't change them. When a transaction comes in, they hash the transaction hash and the secret key for that day, then take the first two bytes of that. Then, a few days later, they publish the secret keys.