PGP
Pretty good privacy for the masses. Phil Zimmermann took a lot of heat from authorities when he gave the world the gift of public key encryption. Like it or not this makes it possible to secure calls, files and email that can be understood by a short list of authorized people. It also allows you to send signed messages so that you can trust that the message was sent by who you think it was.

TrueCrypt
Creates encrypted virtual hard drives for extra security.

Eraser
Delete files securely off your hard drive utilizing DoD standards

ClamWin
Open Source Anti-Virus. Very good at detecting viruses. Can be plugged into hMailServer for e-mail protection LIMITATION: Real Time scanning not implemented yet

 

AVG Anti-Virus
While it is not open source, AVG Free Edition is a solid antivirus program that is available free-of-charge to home users! AVG Free Edition is for private, non-commercial, single home computer use only. There is a commercial version is has more features and allows for central monitoring of your network Here

WireShark
Network Protocol Analyzer, formerly called Ethereal