Written in Python.
Write content with reStructuredText or Markdown.
HTML, or PDF, or ...
Different templates for different output formats. (Can output 1 or more simultaneously.)
Nice for adding math. (via TeX code, presented via MathJAX for web output) Nice for adding live code samples. Can show sync'd input and output. Scripts or console. Python, Ruby, R, etc.
Like Sphinx and other documentation tools, but more power over formatting and customizing.