Eraser, too

Probably just as interesting as TrueCrypt, but more useful, is Eraser, at least for Vista. This utility is also pretty slack when it comes to Human Interface Guidelines, but once you get into it, you can't beat the functionality: Securely delete files, folders, entire volumes, free space on volumes (including cluster tips), the recycle bin, etc. All just a right-click away in the Explorer.

Eraser implements Peter Gutmann's 35-pass file wipe (and several other algorithms, some of them explained a bit in the docs.) Gutmann is the default mode for Apple Macintosh's secure file deletion, source available either from Apple, or from srm.sf.net.