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?
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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.