First, wanted to share an alert about a tricky new form of cyber attack called email conversation thread hijacking that handily thwarts your advice to customers that they should only open messages from people they know. If you are unfamiliar with how it works, you can learn more here.
And without further ado, onto the resources! (I have no known connection with any of these.)
Let's Debug provides a set of tests to help figure out why you might not be able to issue a certificate for Let's Encrypt. You simply enter the domain and validation method to identify problems with basic DNS setup, problems with nameservers, rate limiting, networking issues, CA policy issues and common website misconfigurations. Kindly recommended by Perrydiculous.
The Bash Hackers Wiki provides human-readable documentation and information on GNU Bash so users don't have to read the entire Bash manpage. It's not intended for newbies, but a great resource for those already familiar with Bash. A shout out to whetu for directing us to this one.
A Free Tool
yEd Graph Editor is an application for generating high-quality diagrams either manually or by importing your data. Even large data sets can be arranged by the automatic layout algorithms with a single click. Runs on all major platforms. Thanks go to jmbpiano for the suggestion!
Schneier on Security is a blog on security issues by a fellow and lecturer at Harvard's Kennedy School and a board member of EFF. bdzer0 appreciates it for "high level monthly security/encryption."
Another Free Tool
Hiren's Boot CD includes pretty much everything you need to recover a dead OS or accidentally deleted or lost files. TheGreaterAjax describes it as a "One stop shop for basically anything. From loading antivirus before boot, or imaging a drive that's on it's last leg. Lots and lots of tools built-in, and it has a desktop gui as well."
(You can find a searchable list of the hundreds of items I've shared over time here. Enjoy!)