TrueCrypt - Open Source System Security

TrueCrypt is a Free open-source disk encryption software for Windows 7/Vista/XP, Mac OS X, and Linux. It can create a virtual encrypted disk within a file or encrypt a partition or (under Microsoft Windows except Windows 2000 and Windows 8 with GPT) the entire storage device (pre-boot authentication).

Open Source Security Tool

PeerBlock is an advanced IP blocking utility

PeerBlock lets you control how your PC interacts with other computers over the Internet, letting you block servers and sites known to be troublesome or even dangerous. It lets you choose what to block, and you can also import or create your own lists.

CFEngine - Configuration Management

CFEngine is a popular open source configuration management system, written by Mark Burgess.

PIKT - Open Source Configuration Management

PIKT is cross-categorical, multi-purpose software for global-view, site-at-a-time system and network administration.