dont know of any free ones but you can use the trial version of Tune Up Utilities it has a shredder works really good allows you to overwrite the file up to 999 times. good luck.
CWShredder is a good app to shred the files there is also a applicaiton out there called Eraser which overwrites where the file is stored on the HDD enough times to make sure that there is no chance of recovery of that file.
I have personally used both and still use Eraser to this day. Both are good apps and will get the job done which ever way you want to do it.