Apple Thoughts: Securely Erase Those Files

"When you normally delete your files in Mac OS X, the operating system is only forgetting where those particular files are placed, while the data still physically remains on the drive. Beginning with Mac OS 10.3, Apple enhanced its security by introducing the Secure Empty Trash feature, which follows the U.S. DoD pattern of overwriting data seven times."

Edenwaith created Permanent Eraser and it uses what they say is an even stronger level of deletion by using the Gutmann Method. Basically it overwrites the data 35 times along with some other modifications so that it can't be read via traditional means. This can also erase your CD-RWs and DVD-RWs. It is free Universal Binary and requires OS X 10.3.9 or later.