Hrmm.. I think this has been answered a few times and it is a bit broad and subjective. Depending on what you define as "security" I'd narrow some of it (IMHO) to the following (generic and specific):

  • Windows
    - some kind of Anti-Virus
    - adware/spyware detection tool
    - trojan detection tool
    - firewall (may be on another machine/hardware based)
    - intrusion detection (may be on another machine)
    - auditing tools (retina, nmap, other scanners)
    - backup software/plan

  • Unix/Linux
    - host-based IDS
    - trojan detection
    - firewall (may be on another machine)
    - auditing tools (nmap, nessus, ettercap, netstat, etc.)
    - backup software/plan


That all said, the best security tools is the human brain.

Edit: added backups to both lists