These are other projects I have used or studied, which have similar aims or approaches. (Where dates are not given, descriptions are from around March 2014 or so.)
- tbook
- Very similar to PreTeXt, but pre-MathJax so working very hard on the math. Inactive.
- NoteShare
- Many formats from one source. HTML functionality is much like MathBook’s. Source format is ASCIIdoc. (2014/12/15)
- Text Encoding Initiative
- More about exchange and preservation, less about authoring. More about humanities and less about science.
- DocBook
- Excellent choice for technical documentation, maybe not the right choice for academic work.
- Pressbooks
- WordPress as a front-end to PDF and EBook production. LaTeX support is limited to the WordPress “short code,” which will only create inline snippets as PNG images. (2016-10-19)
- Softcover
- “Frictionless self-publishing.” Authoring language is a unique blend of Markdown and a subset of LaTeX.
- Madoko
- A mix of Markdown and LaTeX processed using the Koka language. Creates both HTML and PDF output. At this writing, no code activity since January 2018. (See GitHub repository.) (2018-08-31)
- GitBook
- “We are building the best tools to help authors publish and sell books.” Proprietary editor, Markdown syntax and “TeX support is planned.” (2014/11/16)
- Ximera
- “Structured LaTeX” as an authoring language. Allows for constructing interactive exercises with data analaysis on student answers.
- TeX4ht
- Best of the converters from LaTeX source. Very steep learning curve to customize or extend.
- TTH
- Another converter from LaTeX. Free to use, but copyrighted with non-standard (weak) license.
- LaTeX2HTML
- Another converter from LaTeX.
- dblatex
- Convert DocBook to LaTeX.
- ReST: reStructuredText
- Lightweight markup, process with Sphinx to various outputs.
- Pandoc
- A “universal” document converter.
- AsciiDoc
- “Text based document generation.” Lightweight markup similar to ReST or Markdown, convertible to multiple output formats.
- Markdown
- Lightweight markup convertible to HTML.
There are probably more. If you think something should be listed here, let me know.