Useful modifiers for solidity

modifiers.sol

// preventing from sending to useless addresses
modifier validDestination( address to ) {
    require(to != address(0x0));
    require(to != address(this) );
    _;
}

// making contract inactive
bool private stopped = false;
address private owner;

modifier isAdmin() {
    require(msg.sender == owner);
    _;
}

function toggleContractActive() isAdmin public {
    // You can add an additional modifier that restricts stopping a contract to be based on another action, such as a vote of users
    stopped = !stopped;
}

// automatic deprecation
modifier isActive() {
    require(block.number <= SOME_BLOCK_NUMBER);
    _;
}