KI6ZHD's Ham page - lots of good stuff especially if you're a UNIX buff

TCP/IP Internet for Amateur Radio

BPQ pilinbpq Source Code
Packet Tutorial - by WB9LOZ - Dated, but the best I've seen
NCPA - Northern California Packet Coordination
International Space Station - How to Connect
WB6YNM's packet page - "Varmint Al"

CW Station Reverse Lookup (CW Skimmers)
Amateur Radio Propagation Beacon Network - Fundamentals
Space Weather - excellent info on propagation influencers

PSK31 - etc.
fldigi - the software I use
PSK Reporter -Station-heard reporting - similar to CW Skimmer only cooler

ISS Information - have a QSO with the Space Station!
Nice write up on using ARISS packet by K9JKM

Software Defined Radio
I recently attended some SDR sessions at Pacificon, and have been completely seduced by the possibilities and directions of SDR. Performance possibilities are beyond anything I'd imagined only a few years ago.

TAPR Support for SDR
Listen Online to a WebSDR

Mesh Network
Broadband Hamnet
How to Load Ubiquiti hardware with Mesh Node Firmware

2015 ARES Manual
Sacramento Valley ARES
Yolo County ARES

RRI and NTS Traffic Handling
NTS Presentation to Yolo ARES
K6HTS's TFC School Videos
Radio Relay International Intro Training
Radiogram Form
ARRL's NTS Methods and Practices Guide - important stuff
Search for ARRL Nets
Northern CA NTS CW Net
Northern CA NTS Slow Speed CW Net
Northern CA, RN7, SSB Net
NTS Message Format - ARRL document
Packet NTS - ARRL how-to
ARRL's NTS Manual - pretty dry reading
WL2K NTSD Batch Files

Main US NTSD Wiki Page
European NTSD Guidelines
NTSD Target Stations
NTSD Parser
ARL Numbered Messages

K6HTN's Traffic School
Nice NTS, NTSD, RadioEmail Summary and Tutorial
Eastern Mass Packet NTS Procedures - a good tutorial
Good Overview of System and WinLink 2000's role - important stuff
W7ARC's NTS Tutorial - easier reading
Central Area Digital Operation Guide
Net Listing - East Bay Section
Morris Co NJ NTS System

Radio Trivia
Big Ear Jansky

Digital Signal Processing
FFTW - The Go-To Fast Fourier Transform Library

ARRL References on RFI/EMI

More links to come...