If you're at a security conference, and you've got a ham radio license (of any class), grab your radio and set it to 146.580 simplex (no offset).
David MacKay has put the textbook he wrote online for everyone to download in a variety of formats. If you find it useful, consider buying a copy.
A lengthy how-to article about building an open source PBX using Linux and Asterisk that uses wireless for its network link. A fully portable wireless telephony system, in other words.
An archive of recorded messages and error tones from United States telephony networks. Available for online listening or free download.
pure-python-otr is just what it says on the tin - it's an implementation of the Off-The-Record messaging protocol in pure Python, meaning that it doesn't hook any libraries that aren't written in Python. Useful for adding OTR support to communications software.
An archive website about coldwar infrastructure in the US. Has documentation about communications systems and facilities.