Markdown was made by John Gruber, and he says:
Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).
The missing Markdown editor for web developers
Haroopad is a markdown enabled document processor for creating web-friendly documents.
MarkdownPad is a full-featured Markdown editor for Windows.
Feature Rich, open and save directly to your blog, github and more, paste images from the clipboard into Markdown, Jekyll Site and Open from folder support plus much more!
MdCharm is a wiki editor, currently it supports markdown(markdown extra) and Multi-Markdown.
Rich editor for Plain text. Write structured documents. Store in plain text. Publish to many formats.
Simple and fast notes on Windows supports Markdown.
Markdown.css — CSS to make HTML markup look like plain-text markdown.
Strapdown.js — Instant and elegant Markdown documents
Markdown Editor plugin for Eclipse — Download it here