@upoker Current "Default" Rules for the regular Bitcoin client (Bitcoin 0.3.20)
0.01 BTC fee if sending any transaction less than 0.01 BTC. This is to help prevent DoS attacks against the network. Remember: fees are not network-enforced, so it's still possible to send these small transactions without the fee -- you just have to generate the blocks that contain them yourself (after modifying Bitcoin).
0.01 BTC fee per kilobyte of transaction, but:
If the blocksize (size of all transactions currently waiting to be included in a block) is less than 27 kB, transactions are free.
If the blocksize is more than 250 kB, transactions get increasingly more expensive as the blocksize approaches the limit of 500 kB. Sending a transaction when the blocksize is 400 kB will cost 5 times the normal amount; sending when it's 499 kB will cost 500x, etc.

search "transaction fees" in wiki
or enter https://en.bitcoin.it/wiki/Transaction_fees