Minimum Tranasction Fees
Bitcoin Core, as of 0.10, has a fee estimation RPC call, which you can read more about here: http://jonathanpatrick.me/blog/floating-fees .

If you don't want to run Bitcoin Core, there are other third party services that provide simple API calls to estimate fees as well, like the Chain Endpoint on BlockCypher's API: http://dev.blockcypher.com/#chain-endpoint (Full disclosure, I work there/am biased)