Darik's Boot And Nuke (DBAN) for destroying data on a whole disk,
Eraser, or multiple free space secure passes in CCleaner to clean "Empty" disk space within a Windows environment,
TrueCrypt for FVE.

Here's a couple of articles:
http://en.wikipedia.org/wiki/Data_remanence
http://www.cs.auckland.ac.nz/~pgut00...ecure_del.html