Links

Tools:

NFSen - http://nfsen.sourceforge.net/
NFDump - http://nfdump.sourceforge.net/
Top 100 Security Tools - http://sectools.org/ 

CERT/CSIRT:

FIRST - http://www.first.org/
CERT/CC - http://www.cert.org/
SANS - http://www.sans.org/
ISC - http://isc.sans.org/

Security Check Lists:

Various Security Checklists -
http://iase.disa.mil/stigs/checklist/

Microsoft Security Checklists and Resource Guides - https://www.microsoft.com/technet/archive/security/chklist/default.mspx?mfr=true

UNIX Computer Security Checklist - http://www.windowsecurity.com/whitepapers/UNIX_Computer_Security_Checklist_.html

Checking UNIX/LINUX Systems for Signs of Compromise - http://www.ucl.ac.uk/cert/nix_intrusion.pdf

Checking Microsoft Windows Systems for Signs of Compromise - http://www.ucl.ac.uk/cert/win_intrusion.pdf

Securing Mac OS X http://www.eecs.umich.edu/dco/security/Securing_Mac_OS_X.pdf