Apps with 'Plain text file' feature
A desktop personal wiki with inline preview as-you-type and lots of plugins.
DokuWiki is a standards compliant, simple to use Wiki, mainly aimed at creating documentation of any kind. It is targeted at developer teams, workgroups and small companies.
Org mode is for keeping notes, maintaining TODO lists, planning projects, and authoring documents with a fast and effective plain-text system.
A Vim-inspired note-taking application that knows programmers and Markdown better.
When you need to create text documents, TextEdit is a highly versatile word processor, including tools to format and layout your page, edit and stylize text, check spelling...