Here's some good sites and ones to have fun with in some cases.
I have found them doing general research to help secure my network and avoid issues. Network security is becoming larger every day as people release scripts so easy the ignorant can use them.SANS Org
- SANS (SysAdmin, Audit, Network, Security) Institute - Tons of materials and links. Great free newsletterCounter Hack Web Site - Author of the Hack-Counter Hack Training Course
often does talks for SANS.The Center for Internet SecurityThe Honeynet Project
read into some of the security papers. For a very hilarious read check out the "Know Your Enemy" series especially the Motives paper where there are links to the irc logs of a group of hackers on a compromised system (honeypot). Little did they know that everything they were doing was being recorded for posterity Insecure.org
home of the very useful utility nmap. Lots of links to security organizations and news lists.SNORT
Home of SNORT. If you don't know what that is you should.
Ok, enough for now. Most of the above will link you to so many other sites that there is no way to adequately categorize it.