The online wallet blockchain.info allows for the fee policy to be customised:
The “Normal” policy states
Follow the fee policy suggested by the mainline bitcoin client.
What is the fee policy suggested by the mainline bitcoin client explicitly?
Answer
https://en.bitcoin.it/wiki/Transaction_fees
Otherwise, the reference implementation will round up the transaction size to the next thousand bytes and add a fee of 0.1 mBTC (0.0001 BTC) per thousand bytes.
So the rate is 0.1 mBTC for each thousand bytes (rounded up) the transaction is.