A sample restricted token, that respects an owner's whitelist.

restricted_token.sol

contract RestrictedToken {
  address owner;

	mapping (address => uint) balances;
  mapping (address => bool) whitelist;

	event Transfer(address indexed _from, address indexed _to, uint256 _value);

	function RestrictedToken() {
		balances[tx.origin] = 10000;
    owner = tx.origin;
	}

  modifier only_owner()
  {
      require(msg.sender == owner);
      // Do not forget the "_;"! It will
      // be replaced by the actual function
      // body when the modifier is used.
      _;
  }

  function reviseWhitelistEntry(address receiver, bool permission) only_owner {
    whitelist[receiver] = permission;
  }

	function sendCoin(address receiver, uint amount) returns(bool sufficient) {
		if (balances[msg.sender] < amount) return false;
    if (whitelist[receiver] == false) return false;

		balances[msg.sender] -= amount;
		balances[receiver] += amount;
		Transfer(msg.sender, receiver, amount);
		return true;
	}

	function getBalance(address addr) returns(uint) {
		return balances[addr];
	}
}