FileVault: It's a built-in feature that provides AES-128 encryption for all the contents in your home directory. Along the same lines, Apple includes Secure Virtual Memory, which encrypts the swap space -- preventing recovery of sensitive information written to swap. While these are more features than applications, they are not enabled by default, and must be configured in the Security pane in System Preferences.

KisMAC and iStumbler: If you do any wardriving, war walking (like Tim enjoys doing around the White House), or surveying wireless networks, these tools will come in handy. They have advanced features like GPS support for mapping, cracking wireless encryption, and detection of wireless networks that are not broadcasting their SSIDs.

VPN client: Most companies have a VPN of some sort that requires a software client or configuration of the OSes' built-in VPN support. I personally use both the Cisco VPN Client and IPSecuritas. Mac OS X supports L2TP over IPSec and PPTP out of the box. IPSecuritas is an IPSec client that supports all the advanced options I needed to get connectivity to my Zyxel firewall at home.

MacPorts: While MacPorts isn’t security-specific, it enables you to install lots of security-related open-source software like nmap and wireshark. It does require, though, that you have the Apple Xcode Tools and X11 installed to compile and use XWindows applications.

VMware Fusion: I use VMware Server on a daily basis to test new security tools and analyze malware. While I could have used Parallels, which currently has a few more features than Fusion, I decided to stick with VMware because of backwards compatibility with virtual machines I've already pre-built for testing.