I write all my documentation in either Confluence, MkDocs (Markdown) or Sphinx (Markdown & reStructuredText).

http://www.mkdocs.org/ http://www.sphinx-doc.org/en/stable/index.html

Both MkDocs and Sphinx can deploy to ReadTheDocs.org which is very nice, especially when using Sphinx since it allows for PDF and ePUB downloads of the documentation as it's versioned.