Rootshell.com is a great site for seeing what exploits are currently
being used by crackers: http://www.rootshell.com/
BUGTRAQ puts out advisories on security issues: BUGTRAQ archives
CERT, the Computer Emergency Response Team, puts out advisories on
common attacks on unix platforms: CERT home
Dan Farmer is the author of SATAN and many other security tools. His
home site has some interesting security survey information, as well as
security tools: http://www.trouble.org
The Linux security WWW is a good site for Linux security information:
Linux Security WWW