user1475: BitCoin Wiki states:

Different bitcoin clients and different versions have different rules for determining which transactions to accept and how large a fee to send..

Is it true that different clients can attach different transaction fees which differ from the other clients?
For example, is there anyway to send 1 mBTC without having to pay the fee of 0.5 mBTC?
Are the rules of the transaction fees enforced by the protocol?

user575: AFAIK there are currently no protocol-enforced rules on transaction fees. There are alternative clients as well as forks of the standard client that allow sending without a fee.
But there is no guarantee that your transaction will be propagated and included in a block. Most of the nodes use the standard bitcoind, so sending a transaction without the fees expected by bitcoind will likely mean your transaction will not be confirmed for some time.

user323: The Bitcoin protocol does not require any fee to be paid.
However, the standard client will require varying fees to be paid before the transaction is included in a block or relayed, all depending on the current load (up to 27kB transactions are free unless they are considered spammy, then fees get higher and higher).
Some pools (like Eligius) don't include transactions without fees into their blocks, so that's another incentive to pay something.

1475: <p><a href="https://en.bitcoin.it/wiki/Transaction_fees" rel="nofollow">BitCoin Wiki</a> states:</p>

<blockquote>
  <p>Different bitcoin clients and different versions have different rules for determining which transactions to accept and how large a fee to send..</p>
</blockquote>

<p>Is it true that different clients can attach different transaction fees which differ from the other clients?</p>

<p>For example, is there anyway to send 1 mBTC without having to pay the fee of 0.5 mBTC?</p>

<p>Are the rules of the transaction fees enforced by the protocol?</p>
 575: <p>AFAIK there are currently no protocol-enforced rules on transaction fees. There are alternative clients as well as forks of the standard client that allow sending without a fee.</p>

<p>But there is no guarantee that your transaction will be propagated and included in a block. Most of the nodes use the standard bitcoind, so sending a transaction without the fees expected by bitcoind will likely mean your transaction will not be confirmed for some time.</p>
 323: <p>The Bitcoin protocol does not require any fee to be paid.</p>

<p>However, the standard client will require varying fees to be paid before the transaction is included in a block or relayed, all depending on the current load (up to 27kB transactions are free unless they are considered spammy, then fees get higher and higher).</p>

<p>Some pools (like Eligius) don't include transactions without fees into their blocks, so that's another incentive to pay something.</p>
