[Last updated December January 19 2017.]
If you have useful security resources that you'd like others to know about, feel free to reply and share them here.
To get us started…
General guides / for multiple groups
[Electronic Frontier Foundation] Surveillance Self-Defense Guide
[Martin Shelton (@mshelton)] Securing Your Digital Life Like a Normal Person
[Martin Shelton] Password Managers for Beginners
[Martin Shelton] Signal for Beginners
[Tactical Tech] HomeApp Centre
[Harlo Holmes (@harlo), Freedom of the Press Foundation] Anti-phishing and Email Hygiene
[Micah Lee, Freedom of the Press Foundation] Security Tips Every Signal User Should Know
[Micah Lee, Freedom of the Press Foundation] Encryption Works
[Computer Incident Response Center Luxembourg] The Digital First Aid Kit
[Digital Defenders Partnership] The Digital First Aid Kit
[Anqi Li & Kim Burton, Access Now] A First Look at Digital Security
[Lorenzo Franceschi-Bicchierai & Joseph Cox] The Motherboard Guide to Not Getting Hacked
[Tactical Tech & Front Line Defenders] Security-in-a-box
[Riseup] Communications Security
[(bluehat)] A Citizen's Guide to Digital Freedoms
[Tactical Tech and Front Line Defenders] Security in-a-Box: Tools and Tactics for Digital Security
[equalit.ie, Transitions Online] Online Learning
[Micah Lee] Encrypting your laptop like you mean it
[Micah Lee] Surveillance Self-Defense Against the Trump Administration
Journalists
[Martin Shelton] Digital Self Defense for Journalists: An Introduction
[Martin Shelton] Source Guide to Defending Accounts Against Common Attacks
[Committee to Protect Journalists] Journalist Security Guide
[Jonathan Stray (@jonathanstray) ] Security for Journalists (Part 1)
[Jonathan Stray] Security for Journalists (Part 2)
[Jose Luis Sierra (Knight ICFJ Fellow)] Salama: Risk Assessment For Journalists and Bloggers
[Tim Jenkin / Center for Investigative Journalism] Operational Security for Journalists (Lecture)
[Center for Investigative Journalism] Information Security for Journalists
[Susan E. McGregor (@susanemcg)] Digital Security & Source Protection for Journalists
[Rory Peck Trust] Digital Security For Freelancers
[Canadian Journalists for Free Expression Journalists in Distress: Securing Your Digital Life
[The Intercept Surveillance Self-Defense for Journalists
Combating gendered abuse
[Tactical Tech] Gender and Tech Resources
[CrashOverride] Coach: Crash Override's Automated Cyber Security Helper
[Feminist Frequency] Speak Up & Stay Safe(r): A Guide to Protecting Yourself From Online Harassment
[Safe Hub Collective] A DIY Guide to Feminist Cybersecurity
Academics
[Marwick, Blackwell, & Lo] Best Practices for Conducting Risky Research and Protecting Yourself from Online Harassment
Resources for security trainers
[Rachel Weidinger, Cooper Quintin, and matt mitchell] Security training resources for security trainers, Winter 2016 Edition
[Internews / LevelUp] Resources for the Global Digital Safety Training Community
[Internews] SaferJourno: Digital Security Resources for Media Trainers
[Tactical Tech] Training Curriculum: Teach others about data and privacy
[equalit.ie] Security Training Curricula (Last updated August 2013)
Activists
[(bluehat)] Privacy Guide for Activists with Haters
[Dia Kayyali, Witness] Getting Started with Digital Security: Tips and Resources for Activists
[Electronic Frontier Foundation] Digital Security Tips for Protesters
[Rory Byrne (@rorybyrne)] How journalists and activists can identify and counter physical surveillance
[Jillian York, Electronic Intifada] A Guide To Online Security For Activists
[WITNESS] A Library of Free Resources for Video Activists, Trainers and Their Allies
[Kade Crockford] Security Culture is Good
Human Rights Defenders
[Association for Progressive Communications] Digital Security First Aid Kit for Human Rights Defenders
Other utilities
[PhishTank] Submit and identify phishing attempts
[Julio Cesar Fort] Public Pentesting Reports
[Internews] Security Auditing Framework and Evaluation Template for Advocacy Groups (SAFETAG)
[r00tz asylum] Whitehat hacking for kids
I'll occasionally update this post to add more.
Resources from this thread have been loosely adapted into a "meta-guide", which can be found here: Current Digital Security Resources.