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?
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.