Security
DShield - http://www.dshield.org/
Distributed Intrusion Detection System firewall system. Provides reports on attack methods.
F-Secure Virus Information Centre - http://www.f-secure.com/v-descs/
Provides a database of viruses with descriptions on each one and what they do.
Return on Information Security Investment - http://www.adrianmizzi.com/
Site that allows you to assess the Return on Information Security Investment for your company.