Here are various open source tools that are useful for security auditing!
If you are a consultant, or an admin that just likes to be on top of his game, these are free open source tools that help audit the security level of workstations and servers in your environment.
Note: These are by nature Linux-friendly apps. I suggest using BackTrack in a virtual machine on your laptop if you do not use Linux in order to easily get access to these (and many more!) tools.
Internal Security Auditing (Linux/POSIX)
Workstation and Software Inventory Management
Network and Packet Analysis
Web Application Testing
Network Discovery Scanner
BackTrack Linux – should be in every security admin’s back of tricks!