Is there a way to not pay the transaction fees when using the standard Bitcoin client? I tried sending one satoshi (0.00000001 BTC) to one of my friend and the client always showed a dialog prompting me to pay 0.0005 BTC in fee, with the only options being OK (pay the fee) or cancel (cancel the transaction). The transaction was using one input and possiby two output (with how the clients jungle with the addresses).

I checked the option and the fee was at 0.00000000 BTC, but the client would not let me placed the transaction.
Yes, however the answer is a bit more advanced than for your everyday consumer:

It is recommended to have some knowledge of JSON if using this method

WORD OF WARNING: Using this method can cause large amounts of inputs being set as miners fees. I suggest you continue with caution and start by using small inputs.

That being said...

A transaction must be compiled and submitted via the bitcoin-qt console using the commands listunspent, createrawtransaction, signrawtransaction and submitrawtransaction.

To access the Bitcoin-qt console:

launch your bitcoin client as usual and wait for it to load the blockchain and start up
click on 'help' in the menu bar (top right)
click on 'debug window'
select the 'console' tab



Use listunspent to get an array of you spendable inputs in your wallet
This is where you set you transaction fee: Use createrawtransaction to generate a transaction in the form createrawtransaction [{\"txid\":\"[txid of the input transaction]\",\"vout\":[vout that relates to the txid of the input transaction]},...] {\"[address to send your bitcoins to]\":[amount of bitcoins to send to this address],...} If you want a no fee transaction you must make sure all inputs equal the output. All inputs that are not spent in a transaction are used as miners fees.
Use signrawtransaction [the output of createrawtransaction] to sign your generated transaction Note: Your wallet must be unlocked to perform this command. If not type walletpassphrase [your passphrase] 21600
Use submitrawtransaction [the output of signrawtransaction] to send your signed transaction to the network to be included into a block


You can see your transaction by going to http://blockchain.info/tx/[txid output from submitrawtrasaction]