You can use Disk Utility. I don't remember when they added the function, but in 10.4, if you open disk utility, and select the PARTITION of the disk that you want to Zero-out free space on (The partition is the child option that has the same name as the disk on your desktop: most likely "Macintosh HD"). Click on the Erase tab, and near the lower left, there will be a button to "Erase Free Space..." which, when you click it, gives you the option to 0-out all data, run a 7-pass erase, or run a 35-pass erase.