Docutils and Sphinx
The biggest inspiration for this project is the pairing of Docutils and Sphinx. They have a similar design: a parser turns text documents into a tree, some transforms are done on the tree (including user plugins), and the output is written to HTML.
My pain points with this system are why I started from scratch. Docutils is an old, old project with less-than-great documentation, verbose code that supports Python 2.4(!), and Sourceforge SVN hosting. reStructuredText is hard to remember and in many cases incapable of producing certain markup. Sphinx supports many documentation use cases out of the box, but its plugin API is limited, its tree processing algorithm makes it difficult to write certain kinds of useful plugins, and its theming system is not straightforward. (I should know, I made a theme.)
Pollen gets a lot of things right. It is more conceptually pure and seemingly more powerful than Computer Words, and if I had the time to grok its docs, I'm sure I would enjoy using it.
But its Markdown mode doesn't let you use custom tags (I think?), so you get to pick extensibility, or a markup syntax you already know, but not both.
Sometimes I can't quite convince myself that I'm not just making a worse version of Pollen using Python. But I honestly think that “native” Markdown documentation is the best way to get contributions from users and buy-in from library maintainers. And reading Pollen's documentation, I just don't get the impression that we really share a target audience.
Butterick's Practical Typography
The author of Pollen is Matthew Butterick, author of Practical Typography. It's a fun and useful read. Many of the defaults in Computer Words are based on its advice. Mr. Butterick feels the same way I do about Sphinx's default themes.
Jekyll is a static site generator that understands Markdown. It is not good for writing your documentation.