ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
TiddlyWiki - a non-linear personal web notebook
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
Scribbleton - The little personal wiki.