Cisco Documentation

ASA Syslog Database
I find this to be very helpful at understanding specific syslog messages generated on the ASA.

IPS Signature Database
This database is useful to further understand a specific signature that is triggering on an IPS.

Partner Education Center
Cisco is often puting together Webex training sessions. Each of these training sessions are recorded and can be found at PEC.

Cisco’s Most Popular Tools
Here you can find things like the bug toolkit, command lookup tool, error message decoder and output interpreter.

Cisco Security Center
Threat information as well as recently disclosed vulnerabilities within Cisco software can be found here.

Security Sites

Secunia
Great site for security advisories and helpful information on staying secure.

Security Tracker
The site aims to keep track of all vulnerabilities that have been disclosed.

Networking Sites

IANA Port Numbers
Port numbers known by IANA.

MX Toolbox
Extremely helpful for troubleshooting mail problems.

Community

Network Engineering on StackExchange
The Network Engineering StackExchange still needs help to be officially launched. Right now there isn’t enough questions asked daily to make it an official site. Come here and ask your question. On average there are 2.6 answers to every question.

Reddit/r/networking
Lot’s of helpful people browse this subreddit. If you have questions about things, these guys can probably help.