Docusaurus - Easy to maintain open source documentation websites
Mailtrack.io - The best email tracking solution I've found. Free
DocFX - A documentation generation tool for API reference and Markdown files!
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Doxygen - Generate documentation from source code
Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.