Gas Cost for keccak256 – What Is the Gas Cost of keccak256(“abc”) in Solidity?

keccak256 (new alias for sha3) is cheapest.

Source: Yellow Paper

Appendix G mentions the gas cost of sha3 is:

• 30 gas + 6 gas for each word (rounded up) for input data to a SHA3 Keccak-256 operation.

Appendix E has the costs for the others.

sha256 (SHA2-256) costs:

• 60 gas + 12 gas for each word (rounded up) for input data to a SHA2-256 operation.

ripemd is even more expensive:

• 600 gas + 120 gas for each word (rounded up) for input data to a RIPEMD-160 operation.

the keccak256() is an alias for the sha3 function to avoid any confusion with the sha-3 standard (The opcode is still called SHA3.) this alias was adopted this month so the old compiler don't recognize it. so use sha3 instead.

The available hashing functions are sha3, sha256, ripemd160:

pragma solidity ^0.4.0;

contract C { 

    function hashingsha3 (string s)   returns  (bytes32 hash){
        return sha3(s);//<=========
    }

    function hashingsha256 (string s)   returns  (bytes32 hash){
        return sha256(s); //<=========
    }

    function ripemd160 (string s)   returns  (bytes20 hash){
        return ripemd160(s); //<=========
    }
}