In main.h, there are 2 lines around line 35 that look like:
static const int64 MIN_TX_FEE = 50000;
static const int64 MIN_RELAY_TX_FEE = 10000;
The first being the minimum fee for a transaction to be included in a block, the second being the minimum fee for one node to forward it to another node. Both numbers are in satoshis.
All of the logic for calculating the fee is in the function GetMinFee() around line 527.
There's a certain area of each block that free transaction are allowed to be in. That number is defined around line 574:
// Free transaction area
if (nNewBlockSize < 27000)
nMinFee = 0;
In the example, 27,000 is the free area (in bytes), where the maximum block size is 500,000. (It's probably a bad idea to set this to 250,000 or higher.)
I believe that the OP was suggesting that you increase the size of the free area, so more feeless transactions can go through.
Whether changing this is a good idea or not is another discussion. Always test on testnet before modifying production servers, don't sue me, etc etc.