A contract that allows sending arbitrary amounts to a list of accounts in an atomic manner

Multisend.sol

//This version does not support sending to contracts with fallback functions. Funds that cannot be delivered are returned to the sender.
contract Multisend { 
    mapping(address => uint) public nonces;
    function send(address[] addrs, uint[] amounts, uint nonce) {
        if(addrs.length != amounts.length || nonce != nonces[msg.sender]) throw;
        uint val = msg.value;
        
        for(uint i = 0; i<addrs.length; i++){
            if(val < amounts[i]) throw;
            
            if(addrs[i].send(amounts[i])){
                val -= amounts[i];
            }
        }
        msg.sender.send(val);
        nonces[msg.sender]++;
    }
}

Multisend2.sol

//This version puts the recipient in charge of withdrawing, avoiding gas cost issues. Recipients must call the "withdraw" function to claim their ETH

contract Multisend2 {
    mapping(address => uint) public balances;
    mapping(address => uint) public nonces;

    
    function send(address[] addrs, uint[] amounts, unit nonce) {
        if(addrs.length != amounts.length || nonce != nonces[msg.sender]) throw;
        uint val = msg.value;
        
        for(uint i = 0; i<addrs.length; i++){
            if(val < amounts[i]) throw;
            
            balances[addrs[i]] += amounts[i];
            val -= amounts[i];
        }
        
        balances[msg.sender] += val;
        nonces[msg.sender]++;
    }
    
    function withdraw(){
        uint balance = balances[msg.sender];
        balances[msg.sender] = 0;
        if(!msg.sender.send(balance)) throw;
    }
    
    function(){
        withdraw();
    }
}

Multisend3.sol

// This one tries sending, and falls back to the withdrawl method if the send fails

contract Multisend3 {
    mapping(address => uint) public balances;
    mapping(address => uint) public nonces;

    
    function send(address[] addrs, uint[] amounts, uint nonce) {
        if(addrs.length != amounts.length || nonce != nonces[msg.sender]) throw;
        uint val = msg.value;
        
        for(uint i = 0; i<addrs.length; i++){
            if(val < amounts[i]) throw;
            
            if(!addrs[i].send(amounts[i])){
                balances[addrs[i]] += amounts[i];
            }
            val -= amounts[i];
        }
        
        if(!msg.sender.send(val)){
            balances[msg.sender] += val;
        }
        nonces[msg.sender]++;
    }
    
    function withdraw(){
        uint balance = balances[msg.sender];
        balances[msg.sender] = 0;
        if(!msg.sender.send(balance)) throw;
    }
    
    function(){
        withdraw();
    }
}

Multisend2.sol has typo: unit instead of uint. I personally like this one best.