A bit of catchup with some useful resources.
The SecLists.Org Security Mailing List Archive is a list of a number of information security and related mailing lists.
The Top 100 Network Security Tools is not a vendor site as such, but a (briefly) annotated list of the most highly regarded (and used) security tools and utilities. An awful lot of these are free. Unfortunately, this is currently based on a 2006 survey, but has been updated in terms of individual tools.
For those teaching, or even seeking to understand, TCP/IP packet headers, a lovely collection of TCP/IP Header Drawings which illustrate the functions quite well. There is no textual explanation;this is not a tutorial or introduction; but as a reminder of some of the most important information, it's great.
Open Source Security Testing Methodology Manual (OSSTMM) security testing or assessment framework. It is interesting that, for an "open source" document, you can only download a partial version, or an old version, unless you are a "gold" member. About half of the Lite 3 version is promotional material, the rest is a checklist of decent, but hardly surprising, checks to perform.
Promiscuous mode, the ability to read all traffic on the network segment even if it's not addressed to you, can be used to mount attacks. It's usually considered a passive attack, because it is used for sniffing. However, there are means to determine if a card on the system is in promiscuous mode.
Port knocking could be used to authenticate requests, but the request and authentication could be observed, and this may be security by obscurity. Even worse, port knocking could be used to set up a covert channel ...
My security frameworks presentation, in PowerPoint. (It's compatible with OpenOffice.) Not just a deck of slides, it has a whole article on the topic embedded in the notes. I used to point at the ISC2 awareness materials, but they seem to change.