A very simple random generator. A miner can influence the number by not publishing a block with an unwanted outcome, and forfeiting the 5 block reward.

Random generator

contract random {
    /* Generates a random number from 0 to 100 based on the last block hash */
    function randomGen(uint seed) constant returns (uint randomNumber) {
        return(uint(sha3(block.blockhash(block.number-1), seed ))%100);
    }

    /* generates a number from 0 to 2^n based on the last n blocks */
    function multiBlockRandomGen(uint seed, uint size) constant returns (uint randomNumber) {
        uint n = 0;
        for (uint i = 0; i < size; i++){
            if (uint(sha3(block.blockhash(block.number-i-1), seed ))%2==0)
                n += 2**i;
        }
        return n;
    }
}